The decision to withdraw U.S. troops from Afghanistan and build up local forces “as quickly as possible” has contributed to “insider attacks,” three senators said Wednesday in a joint statement criticizing the Obama administration’s policies.

On Sunday, four U.S. soldiers were killed in an attack suspected of being carried out by members of the Afghan police; that came a day after two British soldiers were shot dead by an Afghan policeman.

On Monday, officials told NBC News that most joint U.S.-Afghan military operations had been suspended because of the attacks.

“We’re to the point now where we can’t trust these people,” a senior military official said.

US-Afghan military operations suspended after attacks

So far this year, 51 NATO troops have been killed in these so-called blue-on-green attacks.

On Wednesday, Sens. John McCain (R-Ariz.), Joe Lieberman (I-Conn.) and Lindsey Graham (R-S.C.) said in a statement that “in light of the tragic recent attacks on U.S. and NATO forces in Afghanistan, we understand and respect the rationale for scaling back combined operations between coalition and Afghan troops.”

“However, we also believe this decision raises questions about the broader strategy that the Obama administration has been pursuing in this conflict, especially with respect to its timetable for drawing down our military forces in Afghanistan,” they said.

Four US soldiers killed in Afghan 'insider' attack

The statement said the administration had “repeatedly deployed fewer forces than our commanders recommended” over the last three years and “is now drawing down those forces in larger numbers and at a faster pace than our commanders advised.”

“Our military leaders have testified to Congress that these decisions have put our mission in Afghanistan at greater risk, and those risks are now becoming more apparent,” the senators said. “In particular, we are concerned that the rush to build up the Afghan National Security Forces as quickly as possible -- so that U.S. forces could begin withdrawing on the Administration's timetable -- has contributed to the problem of the so-called 'insider attacks'.”

They said President Barack Obama had said the drawdown of U.S. forces would be in response to conditions on the ground.

“We believe those conditions are now worrisome enough to justify an immediate suspension of further U.S. troop withdrawals at this time,” the senators said. “The purpose of this 'strategic pause' should be to give our commanders time to evaluate the effects of recent troop withdrawals and to offer their best military advice on how we can achieve our goals in Afghanistan, while preventing further attacks on our forces and those of our allies. We cannot afford to rush to failure in Afghanistan.”

Responding to the statement, White House Press Secretary Jay Carney said President Obama believes the transition to an Afghan security lead is "absolutely essential" after more than a decade of war.

"We have expended a great deal of blood and treasure in that effort," Carney said Wednesday. "And it is through the heroic and remarkable service of our men and women in uniform in particular that we are at a place now where Afghan security forces have developed capabilities and have developed the numbers that allows them to gradually take over security lead."

Carney added that the green-on-blue attacks are "a very concerning problem," and U.S. officials are working to protect against such attacks, but the transition process will not be affected.

NBC's Libby Leist and Ali Weinberg contributed to this report.

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Abdul Khaleq / AP

An Afghan policeman checks a man entering the police station near the gate of a joint civilian-military base where two British soldiers part of the NATO forces were killed south of Kabul in Afghanistan.

Updated at 12:26 p.m. ET: Three NATO troops were killed in Afghanistan on Monday in two separate attacks apparently carried out by members of local security forces.

A gunman wearing an Afghan army uniform killed two NATO troops in southern Afghanistan, while another was shot in eastern Afghanistan by an alleged member of the Afghan Local Police.

The deaths appeared to be the latest in a string of so-called "green on blue" attacks in which Afghan security forces have turned on their international colleagues or mentors.

The incident in which two died took place in Lashkar Gah in the southern Helmand province, an Afghan security source told Reuters. It will be investigated by a joint Afghan-NATO team.

"The individual who opened fire was killed when coalition forces returned fire," NATO's International Security Assistance Force (ISAF) said in a statement about that incident. It did not identify the two NATO soldiers or give their nationality.

The Lashkar Gah base is dominated by British forces and a NATO official told The Associated Press the two victims were British soldiers. The official spoke on condition of anonymity because the information had not yet been formally announced.

Britain's Ministry of Defense said it was aware of the Lashkar Gah incident, but refused to release any more details or confirm whether the soldiers were British.

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In a later statement about the shooting in eastern Afghanistan, ISAF said: :According to operational reports, the ISAF service member was shot by an alleged member of the Afghan Local Police as the security force approached an ALP checkpoint. The circumstances behind the incident are currently under investigation by Afghan and coalition forces."

There were no further details immediately available.

Insider attacks have mounted in recent months as tension between Afghanistan and its foreign backers rises over a series of incidents including the burning of Qurans at a NATO base and the killing of 17 villagers in southern Kandahar for which a U.S. soldier has been charged.

Before Monday's attack, 13 members of the NATO-led force had been killed this year in what appeared to be attacks by members of Afghan forces, the commander of U.S. and NATO forces, General John Allen, told a U.S. Senate committee last week.

About 70 members of the NATO force have been killed in 42 insider attacks from May 2007 to January this year.

The shootings raise new concern about the reliability of Afghan forces and their ability to take over security responsibilities by the end of 2014, when most Western combat forces leave.

Reuters, The Associated Press, NBC News' Cheryll Simpson and msnbc.com staff contributed to this report.

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