It could be a long, hot and deadly summer for American forces in Iraq, who may soon face increased attacks, Defense Secretary Donald H. Rumsfeld (searchsays.

He also acknowledged the Bush administration does not know how long the U.S.-led occupation will last or how much it ultimately will cost.

American and coalition troops have been the target of daily sniper shootings, rocket-propelled grenades and other attacks.

Just hours after Rumsfeld's warning, one American soldier was killed and six others wounded early Monday in an attack by insurgents who fired rocket-propelled grenades at their convoy, a military spokesman said. The military also said a marine in southern Iraq died in a non-hostile incident. It provided no details.

Since May 1, when President Bush (search) declared major combat over, more than 30 U.S. troops have been killed and scores wounded in hit-and-run attacks.

"I'm afraid we're going to have to expect this to go on," Rumsfeld said Sunday on NBC's Meet the Press.

"And there's even speculation that during the month of July, which is an anniversary for a lot of Baathist (Saddam Hussein's party) events, we could see an increase in the number of attacks," he said.

The defense secretary declined to use the term "guerrilla war" to describe the current situation.

He said there is still a lot of debate about whether the resistance American forces are encountering is organized throughout the country. However, he said, "it's very clear that it's coordinated in regions and areas, cities in the north particularly."

Rumsfeld also disputed claims by some congressional Democrats that the administration has understated the cost of the war and occupation of Iraq.

"We have said we don't know what it will cost. We have said it's not knowable how long it will last," he told ABC's This Week.

Rumsfeld said estimates he provided Congress last week that the occupation was costing $3.9 billion to $4 billion a month are based on current costs and cannot be projected into the future.

There are about 150,000 U.S. troops in Iraq. Rumsfeld would say only that they may be there for the "foreseeable future," and the number could be increased if necessary.

He dismissed concerns that the United States may be getting bogged down in Iraq and he vowed to stay the course.

"Is it an important thing to be doing? Yes. Is it tough? You bet. Are more people going to be killed? You bet. Does it cost some money? You bet. Can we tell the world or anybody else precisely what it's going to cost or how long it's going to last? No," he said.

Former NATO supreme commander Gen. Wesley Clark, who is considering a run for president, said Monday that because the U.S. armed forces are "overstretched because of Iraq," it will be difficult for American intervention in other strife-torn nations, including Liberia.

U.N. Secretary-General Kofi Annan has been urging the White House to send military assistance to that west African country.

"We need to take measures to take care of the men and women in uniform. They need reserves called up, they need a rotation plan, because let's face it, we're going to have sustain the force in Iraq for some time," Clark said on television.

Rumsfeld also said he's confident that weapons of mass destruction (search) will be found in Iraq.

"We just have to be patient," he said. "It's been 10 weeks now. We've got a wonderful team of people working on the problems ... and they're going to keep looking."

Sen. John Kerry, D-Mass., a presidential hopeful, said the administration should move quickly to bring other countries into the postwar effort in order to take the focus off American troops.

"The obligation of the United States government and the president is to rapidly internationalize the effort in Iraq, get the target off of American troops, bring other people, particularly Muslim-speaking and Arabic-speaking Muslim troops, into the region," Kerry said on a TV news program.

Rumsfeld said the United States is doing just that. "We have got 19 countries already assisting. We have another 19 countries that are agreeing to assist and another 11 that we're talking to," he said. "So, it's a very large international coalition."

On Monday, the Indian government rejected a U.S. request for Indian peacekeeping troops in Iraq, saying such a deployment would be considered only under a U.N. mandate.

Despite the security concerns, national security adviser Condoleezza Rice insisted that progress is being made in Iraq.

"Oil is beginning to flow again. Electricity is being repaired. Police are being trained. Universities are being opened. And not surprisingly, the old Baathists are trying to attack that success," Rice said.