Panetta in Afghanistan amid tensions over shooting deaths

Defense Secretary Leon Panetta arrived in Afghanistan on Wednesday to meet with military commanders and local leaders, days after a U.S. service member allegedly went on a shooting rampage that left 16 Afghan civilians dead.

The Obama administration is working to calm tensions after the rogue attack, and Panetta is expected to reassure Afghan President Hamid Karzai that the United States will conduct a full investigation. 

Panetta told troops at Camp Leatherneck in Afghanistan that recent incidents there have been "deeply troubling," but would not change the mission.

"We will not allow individual incidents to undermine our resolve," Panetta said, according to AFP. "We will be tested, we will be challenged by the enemy, by ourselves and by the hell of war itself."

The Associated Press reported that before Panetta spoke, the more than 200 Marines attending were asked to remove their weapons from the large tent, an unusual command. "Something has come to light," Sgt. Maj. Brandon Hall told the troops. 

A defense official told the AP that the request was made out of respect for soldiers from other countries, like the Afghans, who are never allowed to bring weapons into events.

Panetta's trip, which reports say had been planned months in advance but was unannounced, has acquired a new sense of urgency, as the White House hopes to quickly repair relations with the Afghan government.

Karzai has called the civilian deaths “an assassination” and said they “cannot be forgiven,” in a statement released Sunday. Some Afghan lawmakers have called for the American service member suspected in the attack to be tried in Afghanistan. 

President Obama called Karzai on Sunday to offer his condolences. 

“The United States takes this as seriously as if it was our own citizens and our own children who were murdered,” Obama said during brief remarks Tuesday during an event in the Rose Garden. “We are heartbroken over the loss of innocent life.”

The president met earlier Tuesday with Gen. John Allen and Ambassador Ryan Crocker in Washington and said he's ordered the Pentagon to "spare no effort" in the investigation.

The incident came on the heels of a wave of violence last month, sparked by the accidental burning of Qurans at a U.S. air base. The subsequent protests led to the deaths of six U.S. soldiers and over 30 Afghan civilians.

The White House is now facing increased pressure from some Republican and Democratic lawmakers to accelerate the pullout of American forces from Afghanistan. 

Twenty-four senators, including two Republicans, signed a letter last week calling for the timetable for withdrawal to be brought forward.

The administration has said, however, that the tragic incident this weekend has not changed U.S. strategy in Afghanistan and denied reports that the timetable for withdrawing forces from the country had been altered.

White House press secretary Jay Carney on Tuesday said the post-surge drawdown would “be decided in consultation with NATO ministers and will have everything to do with the successful implementation of the strategy.”

Senate Minority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellGOP strategist donates to Alabama Democrat McConnell names Senate GOP tax conferees Brent Budowsky: A plea to Alabama voters MORE (R-Ky.) on Tuesday offered his support, saying he backed the current scheduled withdrawal.

Acknowledging that the United States faced a "very challenging period," McConnell called on the White House to "stick with the plan that's been laid out."

Fellow Republican Sen. John McCainJohn Sidney McCainGOP strategist donates to Alabama Democrat Meghan McCain knocks Bannon: 'Who the hell are you' to criticize Romney? Dems demand Tillerson end State hiring freeze, consult with Congress MORE (Ariz.), however, slammed the decision to proceed with withdrawing 23,000 troops by the end of the year. McCain said that the drawdown "discourages our friends and encourages our enemies.”

The suspect in Sunday's tragic attack is reported to be a U.S. Army staff sergeant and is being held in custody on a NATO base.

On Monday, Panetta said the death penalty was possible for the suspected attacker.

The Defense secretary is not expected to visit the town where the deaths occurred. On Tuesday, an Afghan delegation to the province was attacked by militants.

This story was updated at 9:09 a.m.