Increasing congressional discord over the next U.S. steps in Afghanistan, coupled with a spike in violence there, is deepening the political divide on the war and how many troops are needed to fight it.

Key Senate Democrats signaled Friday that any push by the White House to send more troops to Afghanistan is likely to hit resistance. And their unease was fueled by another bombing, that left as many as 70 dead, including civilians who were killed when the U.S. blew up tanker trucks hijacked by the Taliban.

That deadly U.S. airstrike in Afghanistan Friday complicates the debate over the need for more U.S. troops, bolstering arguments that Afghan leaders must increasingly fend for themselves.

Senate Armed Services Committee Chairman Carl Levin said the United States must focus more on building Afghanistan's security forces. His cautionary stance was echoed by Sen. Jack Reed, who is also on the committee and spent two days in Afghanistan this week with Levin.

The senators will return to Washington next week, just as President Barack Obama receives a new military review of Afghanistan strategy that officials expect will be followed up by a request for at least a modest increase in U.S. troops battling insurgents in the 8-year-old war.

Obama came into office pledging to shift U.S. focus from the war in Iraq to the Afghan fight, which had long been a secondary priority. But as war-weary Americans have watched another 21,000 troops go to Afghanistan this year, and U.S. casualties rise, support for the war has waned.

As a result, lawmakers say they want the U.S. to more quickly train and equip the Afghan Army and police so the embattled country can take over its own security needs.

"There are a lot of ways to speed up the numbers and capabilities of the Afghan army and police. They are strongly motivated," Levin said from Kuwait. "I think that we should pursue that course ... before we consider a further increase in combat forces beyond what's already been planned to be sent in the months ahead."

Levin said there is a growing consensus on the need to expedite training and equipping the Afghan army to improve security in Afghanistan, where 51 U.S. troops died in August, making it the bloodiest month for American forces there since the U.S.-led invasion in late 2001.

Still, the airstrike comes just as U.S. defense leaders insisted this week that troops were making great progress in stemming civilian deaths.

"All I can really do is assure you that they recognize the gravity of these events when they happen and it has the full attention of the leadership," Defense Department spokesman Bryan Whitman said of forces in Afghanistan that were investigating the Friday attack.

Afghan and German officials said a total of 50 to 70 people died early Friday morning after German forces called in an American airstrike on Taliban insurgents who had hijacked two fuel tankers in northern Kunduz province. There were reports that some of the dead were civilians who swarmed around the trucks to siphon fuel.

Under new orders put in place by top commander Gen. Stanley McChrystal in July to reduce civilian casualties, U.S. and NATO forces were directed to limit air support to ground troops when civilians might be present.

And Adm. Mike Mullen, Chairman of the Joint Chiefs of staff, told Pentagon reporters this week that Afghan civilian casualties were being greatly reduced.

In a separate call with reporters Friday, Reed, a Rhode Island Democrat, said the U.S. must use a multi-pronged approach to win the war in Afghanistan. The U.S., he said, must build up the Afghan Army, send more civilians to Afghanistan to provide economic and political assistance, and reach out to Taliban supporters who are willing to recognize the Kabul government.

The hesitancy to boost troops levels comes just days after Obama's defense chief suggested a willingness to consider an increase. Defense Secretary Robert Gates this week urged patience with the war effort, and said he would be comfortable with a larger U.S. military presence in Afghanistan as long as the increase reassured the country's citizens that the Americans were there for the benefit of Afghans.

Gates has declined to talk about any specific recommendations contained in a new review of Afghanistan strategy presented this week by McChrystal.

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Associated Press writers Pauline Jelinek in Washington and Eric Tucker in Providence, R.I. contributed to this report.