As U.S. and Iraqi leaders opened talks this week to discuss an American troop withdrawal, one of the biggest one-week death tolls for U.S. forces in Iraq and a continuing surge in killings of Iraqi forces and civilians showed that the insurgency is increasing its lethality and expanding its scope, according to U.S. and Iraqi officials and casualty counts.

Of the 29 U.S. combat fatalities this week, 14 were U.S. Marines who died Wednesday when a three-high stack of antitank mines exploded under their assault vehicle. The seven-day period ending Saturday night was the Americans' fourth-deadliest week in combat since the invasion of Iraq in March 2003, based on statistics compiled by the Defense Department and by the Iraq Coalition Casualty Count, an independent Web site.

The steadily mounting U.S. combat losses suggest that Iraq's two-year-old insurgency remains capable of repeatedly inflicting significant losses on the Americans. Meanwhile, the guerrillas have broadened their Iraqi targets to include Shiite Muslim civilians and politicians, in a bid to provoke full-scale civil war.

Last month, Gen. George W. Casey Jr., the top U.S. commander in Iraq, described the insurgency as stagnating and outlined the possibility of starting a U.S. troop withdrawal next spring. The top military spokesman in Iraq, Brig. Gen. Donald Alston, told reporters in Baghdad on Thursday: "This is not an expanding insurgency. What we're seeing is probably the opposite."

Yet combat deaths among U.S. forces have greatly increased in the second half of the war and continue at a steady pace. Last month, for example, U.S. and allied security forces reported 39 deaths from bombings, the highest monthly toll since the war began, according to the Iraq Coalition Casualty Count, which compiles its statistics from Defense Department data.

There are an average of 70 attacks a day on U.S. forces, Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, said in a July 21 statement he prepared for a government court case. "As I have publicly stated, our assessments indicate that the lethality of the attacks is on average increasing," Myers wrote.

Myers also cited "documented near-term increases in the assassination of Iraqi government officials (52 in the three-month period ending June 27, 2005), as well as a recent uptick in insurgent attacks on senior diplomatic officials from regional neighbors of Iraq."

In addition, Iraqis are sustaining many more losses than Americans. The number of deaths among Iraq's military and police forces has tripled between January and July, according to figures compiled by the Washington-based Brookings Institute from news media and official reports. The institute's Iraq Index shows the killings surging from the low 100s in January and February to about 200 in March and April to 259 in May, 296 in June and 304 in July.

Figures for war-related killings of Iraqi civilians are imprecise because not all news of civilian deaths reaches the media. But according to Brookings, roughly 400 to 700 Iraqi civilians were killed in July -- a decrease of 100 to 200 from June. The number of dead was still at least 100 more than in April.

The past week's surge in American combat deaths came as a top-level Iraq-U.S. commission held its first meeting in Baghdad to plan for a U.S. withdrawal. Iraq's national security adviser, Mowaffak Rubaie, said commission members were slated to give Prime Minister Ibrahim Jafari their recommendations in 60 days on what security conditions should be met before any significant reduction of the 138,000 U.S. troops in Iraq could begin.

Conditions under discussion include the level of strength, training, equipping and command of the Iraqi security forces, U.S. and Iraqi officials said. Casey said last month that substantial withdrawals could begin by March or April if Iraq's political process and security force buildup were on track.

Iraqi leaders, more than their American counterparts, stress the importance of reining in the insurgency ahead of a U.S. pullout.

"There are so many variables," Rubaie said Saturday. "What is the level of the insurgency, of terrorism? What is the position of regional countries" accused of harboring insurgents?

On Thursday, Alston, the military spokesman, cited the continuing buildup of Iraqi troops and what he described as successes against the insurgency. Asked for evidence, he said 13 vehicle bombings were recorded last week, three of them suicide attacks. He said both figures were the lowest since April.

"When I look at the bar charts, it's a clear indication to me that the tempo of suicide attacks has decreased," Alston said.

Military spokesmen said at the time that they would provide overall figures on vehicle, suicide and roadside bombings. The figures had not been released by late Saturday.

U.S. and Iraqi leaders say Iraqi-led raids in Baghdad and U.S.-led raids in the western province of Anbar -- the scene of the past week's heaviest losses -- have disrupted insurgents' command and supply lines and reduced the number of bombings.

The capture or killings of numerous lieutenants of the insurgent leader Abu Musab Zarqawi and others also are believed to have thwarted specific attacks, commanders say. Insurgent groups have been forced to reorganize to make up for the leadership losses, with many moving under the umbrella of al Qaeda in Iraq, Zarqawi's group, according to some Western intelligence officials and insurgents.

But the U.S. military says its enemy is adaptive: Perhaps because crackdowns have disrupted supply lines for bombs, small arms appear to be used more. When U.S. forces sweep regions like Anbar, troops and residents say, insurgents return as soon as the Americans leave.

Additionally, the U.S. military presence here has helped attract radical young Muslim men from inside and outside Iraq. American military officials last year put the number of insurgents as low as 3,000; official estimates last month ranged from 16,000 to 20,000, including perhaps 1,000 foreign fighters.

Iraqis and Americans blame the foreign fighters for introducing suicide bombings to Iraq, terrorizing daily life in Baghdad and other cities and solidifying most public opinion against the foreign extremists.

But bombings have also achieved insurgents' aim of reducing support for Jafari's interim government. Public criticism of Jafari's government as ineffective grows when the insurgents often-cyclical bombing campaigns are on the upswing.

As the political process evolves, shifting power to Iraq's Shiite Muslim majority, the Sunni Arabs who make up the bulk of the insurgency have focused with increasing frequency on attacking Shiites and political figures -- a shift that insurgent leaders say came from the top down, starting with Zarqawi.

"We fight the Americans and the Shiites," said a 55-year-old Iraqi insurgent leader in Ramadi who calls himself Abu Hasan Ansari. "But because the Shiite danger to Islam became bigger than the American danger, the priority is to fight the Shiites before the Americans."

Staff writer Josh White in Washington contributed to this report.