KABUL — Bombs and bullets killed seven American troops on the deadliest day for U.S. forces in Afghanistan in nearly a year -- and a sign that the war being fought in the Taliban heartland of the south and east could now be expanding north.

Four of the deaths Monday came in an attack on a team of U.S. military trainers in the relatively peaceful north, bringing into focus the question of whether the U.S. is committing enough troops to secure a country larger than Iraq in both population and land mass.

Separately, Taliban militants claimed on a militant Web site that they were holding an American soldier whom the U.S. military says insurgents might have captured last week. The Taliban statement, however, did not include any proof, such as a picture or the soldier's name.

On a visit to Moscow, President Barack Obama said it's too soon to measure the success of his new strategy in Afghanistan. He said the U.S. can take another look at the situation after the country's presidential elections on Aug. 20.

Russian President Dmitry Medvedev said in some respects, progress has been "insignificant" in Afghanistan. He said it's hard to say how quickly the situation will improve.

Obama has ordered 21,000 additional American troops to this country, mainly in the south where Taliban militants have made a violent comeback after a U.S.-led coalition topped them from power in late 2001. The U.S. expects 68,000 troops here by year's end, double last year's total but still half as many as now in Iraq.

The four American soldiers killed in the north died in a roadside bombing of their vehicle in Kunduz province, said Navy Chief Petty Officer Brian Naranjo, a U.S. military spokesman. The soldiers were training Afghan forces, he said.

Two Americans were killed in a roadside blast in southern Afghanistan, Naranjo said. And another American soldier died of wounds in a Monday firefight with militants in the east, a U.S. military spokesman said.

There were no further details on the incidents in the south and the east.

It was the deadliest day for American troops in Afghanistan since July 13, 2008, when 10 soldiers were killed -- nine of them when militants using small-arms fire and rocket-propelled grenades attacked a remote outpost in the village of Wanat near the Pakistani border.

The Taliban claim about holding a captured U.S. soldier came six days after a soldier was noticed missing during a routine check of his unit June 30. His body armour and weapon were found on the base.

Two U.S. defence sources have said the soldier "just walked off" post with three Afghans after he finished working. They had no explanation for why he left.

In southern Afghanistan, meanwhile, thousands of U.S. Marines continued with their anti-Taliban offensive in Helmand province. Some 500 Marines out of 4,000 participating in the operation moved into the Khan Neshin area, a Marine statement said Monday.

"This is the first time coalition forces have had a sustained presence so far south in the Helmand River valley. Khan Neshin had been a Taliban stronghold for several years before Afghan, and coalition forces arrived and began discussions with local leaders several days ago," the statement added.

In the southern province of Kandahar, meanwhile, a suicide car bomber blew himself up outside the outer gate of the main NATO base in the region, killing two civilians and wounding 14 other people.

Those wounded near the gates of Kandahar Airfield included 12 civilians and two Afghan soldiers, said Gen. Sher Mohammad Zazai, the top military commander for southern Afghanistan.

As the conflict intensifies, U.S. forces are under pressure to minimize civilian deaths in military operations. In an effort to reduce civilian losses, the new commander of U.S. and NATO forces, Gen. Stanley McChrystal, issued guidelines ordering troops to "scrutinize and limit" the use of airstrikes against residential compounds, which Taliban fighters often use as hideouts.

McChrystal says he hopes to produce a cultural shift in the military so that his troops' first priority will be protecting Afghan civilians, not using massive fire power. McChrystal's guidelines went into effect last week, and officials released a declassified version Monday.

The three directives are that airstrikes must be authorized and very limited but can be used in self-defence if troops' lives are at risk; troops must be accompanied by Afghan forces before they enter residences; and troops cannot go into or fire upon mosques or other religious sites, though this is already U.S. policy.