U.S. military officials said Friday that American help for Syrian rebels is likely to escalate gradually, beginning with basic equipment like body armor and night-vision goggles and shifting later to weapons and ammunition.

The officials told NBC News that providing the rebels with heavy weapons, such as anti-tank or anti-aircraft rockets, was not being actively considered. The gradual timetable could be accelerated if circumstances change, the officials said.

The details came a day after the White House said that it believed the government of Syrian President Bashar Assad had used chemical weapons, including the nerve agent sarin, against the rebels.

As a result, President Barack Obama decided to provide “military support” to a major opposition group in Syria, Ben Rhodes, the deputy national security adviser, told reporters.

The military officials told NBC News that the timing of the support was the decision of the White House and State Department, with the Defense Department providing some material and delivering it to the region.

The U.S. military could also provide the rebels with strategic and tactical combat training, most likely in Jordan, where some combat elements are already positioned for a previously scheduled exercise.

A Patriot missile defense system and as many as eight F-16 fighter jets will likely remain in Jordan following the exercise. Elements of a Marine Corps expeditionary force off the helicopter carrier USS Kearsarge are expected to arrive in Jordan soon.

Defense officials stressed that there was no consideration being given to using American ground forces. For now, they also rule out imposing a no-fly zone.

Earlier Friday, Syria said that the United States was lying about the regime's use of chemical weapons, while Russia called the claims unconvincing — a dramatic turn in the two-year conflict.

"The United States, in resorting to a shameful use of pretexts in order to allow President Obama's decision to arm the Syrian opposition, shows that it has flagrant double standards in the way it deals with terrorism," Syria's foreign ministry said.

Syria has maintained that "terrorists" are using the chemical weapons.

Russia, which has opposed sanctions and vetoed U.N. Security Council resolutions to put pressure on Assad, reacted with skepticism to the White House's announcement.

President Vladimir Putin's senior foreign policy adviser said Friday that the information the U.S. has "does not look convincing."

Yuri Ushakov said more U.S. military support for Assad's opponents would undermine joint efforts to bring together Syrian government and opposition representatives for peace talks. 

According to the United Nations' human rights office, the two-year-old war in Syria has killed almost 93,000 people, although it says the real number is likely to be much higher.

The United Kingdom, which says it provided evidence of chemical weapons use in Syria to a United Nations investigation, had not decided whether to arm the rebels, a government spokesman said Friday.

"Nothing is off the table," the spokesman said, adding that the U.K. was "in urgent discussions with [its] international partners."

On Wednesday, British Prime Minister David Cameron said he was eager to host the G8 summit next week in Northern Ireland. "We should use the G8 to try and bring pressure on all sides to bring about ... a peace conference, a peace process, and a move towards a transitional government in Syria," he said.

In an interview with the BBC on Friday, German Chancellor Angela Merkel urged the UN Security Council to "achieve a united approach."

But France raised the concern that a Security Council resolution, such as the establishment of a no-fly zone over Syria, would face opposition from some members. 

Slideshow: Syria uprising

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A look back at the conflict that has overtaken the country.

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"The problem with this type of measure is that it can only be put in place with approval from the international community," French Foreign Ministry spokesman Philippe Lalliot said Friday.

NBC's Albina Kovalyova and Reuters contributed to this report.

Related stories:

• US offers Syrian rebels 'military support,' alleges Assad used chemical weapons
• First thoughts: What's the endgame for Syria?
• 'Long overdue': Reactions to White House announcement on Syria
• Analysis: A battle may be won, but war will rage on for Syria's Assad

President Obama said Friday that the potential use of chemical weapons by the ruling regime of Syria against its people “adds increased urgency” to international concern about the regime.

Speaking to reporters during an Oval Office meeting with King Abdullah of Jordan, Obama noted that reports of the use of chemical weapons by the Syrian government were preliminary. That information, he said, “does not tell us when they were used, how they were used.”

Still, the president said: “Obviously, horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law. And that is going to be a game-changer.”

On Tuesday, the Israeli military published intelligence findings that President Bashar Assad’s forces had used chemical weapons repeatedly in recent months. Part of Israel’s concern, and Obama’s, is that those weapons could fall into terrorist hands.

Two days later, Defense Secretary Chuck Hagel said that the U.S. believes “with some degree of varying confidence” that the Syrian government has used chemical weapons, specifically the nerve agent sarin, against its people.

A letter from the White House to Congress said the assessment was based on “physiological samples” but called for a United Nations probe to corroborate it and nail down when and how they were used.

The American response is shadowed by the legacy of flawed intelligence reports of weapons of mass destruction that led to the invasion of Iraq.

The president spoke after the deputy foreign minister of Israel said world powers may now conclude there was “no avoiding” action to take control of the Assad regime’s chemical stockpile.

British Prime Minister David Cameron also said there was limited but growing evidence that the Syrian regime had used chemical agents.

Echoing the administration’s caution, Rep. Dutch Ruppersberger of Maryland, the top Democrat on the House Intelligence Committee, said Friday that “every option is on the table” but stressed that “we want to do everything we can to avoid putting boots on the ground.”

Sen. John McCain, R.-Ariz., said on Thursday that the Obama administration should consider a military approach but not commit American troops. He suggested providing weapons to trusted parts of the Syrian resistance.

The uprising against Assad began in March 2011, and an estimated 70,000 people have been killed in the violence that has followed.

Slideshow: Syria uprising

/

A look back at the conflict that has overtaken the country.

Launch slideshow

Related:

No good options for Obama on Syria

Bush admin's Iraq WMD claims hang over Syria chemical weapons debate

'Suffocating in the streets': Chemical weapons attack reported in Syria

This story was originally published on Fri Apr 26, 2013 8:02 AM EDT