It's not going to be a peaceful summer in Iraq for US forces, who may face increased attacks in the coming days.
The stark warning came Sunday from US Defense Secretary Donald Rumsfeld, who also acknowledged the administration does not know how long the 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. Since May 1, when US President George W. Bush declared major combat over, 31 US soldiers 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 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 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 situation on the ground.
He said there is still a lot of debate about whether the resistance US 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 from 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 said on ABC's This Week.
Rumsfeld said estimates he provided Congress last week that the occupation was costing US$3.9 billion to US$4 billion a month are based on current costs and cannot be projected into the future.
There are about 150,000 US troops in Iraq. Rumsfeld would only say that they may be there for the "foreseeable future," and he said that number could be increased, if that is what is needed.
Rumsfeld dismissed concerns that the US 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.
Rumsfeld also said he's confident the US will find weapons of mass destruction 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."
Democratic presidential hopeful John Kerry 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 CNN's Late Edition.
Rumsfeld said the US 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."
Despite the security concerns, Bush's national security adviser, Condoleezza Rice, insisted that progress is being made in Iraq.