Retired US General Wesley Clark said on Wednesday that President George Bush's approach to Syria and Iran encourages the two countries to work against US interests in Iraq and endangers overstretched US troops there.
Basic military strategy
"If you want to succeed in Iraq, you should isolate the battlefield. That's a basic rule of military strategy," Clark told the US House of Representatives Armed Services Committee.
The former top Nato general ran unsuccessfully for president last year and has taken a leading role among Democrats as a critic of US policy in the Middle East.
Former Pentagon adviser Richard Perle rejected the idea of reassuring Iran and Syria, saying the two countries support terrorism and fear US success in Iraq because it threatens their governments.
"We're going to destabilise at a greater rate than we can stabilise. I think this committee should be holding hearings right now on what the United States should do if Syria collapses"
Retired General Wesley Clark
"That's the last thing we should be doing," he said. "We will get no help from countries whose interests are diametrically opposed to our own."
Bush has hammered Iran on its support for "terrorist" groups and its nuclear ambitions and pushed Syria's Baath Party government to withdraw from Lebanon.
Bush administration officials have criticised both countries for aiding fighters in Iraq.
Clark told lawmakers he was concerned the administration's rhetoric would lead to the collapse of Syria's government, saying Washington was unprepared to handle that possibility.
"We're going to destabilise at a greater rate than we can stabilise," he said.
"I think this committee should be holding hearings right now on what the United States should do if Syria collapses, because that's clearly the policy."