He said that an attack on Iraq would open a "Pandora's Box" while the Israel-Palestinian conflict remained unresolved.
The king's comments, in an interview with the UK's Times newspaper, come amid increasing speculation that the US is planning to topple Iraqi President Saddam Hussein.
The prospect of US military action in Iraq has polarised international opinion and is now reportedly facing opposition at home as well as abroad.
The Washington Post newspaper says some senior US military officials favour the present policy of containment plus sanctions towards Iraq, rather than any high-risk attempt to topple the Iraqi leader.
No decision
Before flying to Washington on Monday, the Jordanian monarch - one of America's staunchest Arab allies - held talks in London with UK Prime Minister Tony Blair.
Mr Blair's office said the two men had focused on the need for progress in the Middle East peace process, but had also touched on the need to address the problems posed by Iraq's stance on weapons of mass destruction.
Mr Blair's spokesman stressed that Britain was not at the point of making decisions about a possible American invasion of Iraq.
"Nothing has been decided by the US administration as to how this issue is to be taken forward," the spokesman said.
Hawks 'obsessed'
In his remarks, the king expressed concern about hawks in the US administration who were "fixated on Iraq".
He said if they held sway, "that really would destabilise American strategic interest even more in the Middle East".
The monarch said the international community was united in its opposition to an attack on Iraq.
"All of us are saying: 'Hey, United States, we don't think this is a very good idea.'"
The king denied reports that Jordan would host US troops preparing to bomb Iraq.
Disagreement over Arafat
The king criticised President Bush's call last month for Palestinian leader Yasser Arafat to be replaced.
He said the American strategy would backfire.
"It is unfortunate to target any leadership in any part of the world," he told the newspaper.
"The majority of Palestinians, who feel that America is biased, simply to spite the Americans would probably vote for Arafat to show 'this is what we think of what you said'."
He warned against exerting pressure on Mr Arafat, saying the Palestinian leader's "popularity goes up the minute you point an Israeli tank barrel at him or there is a statement that comes out of Washington".
The king said he felt recent Palestinian moves to stop suicide bombings were genuine.
He said he believed the radical Islamic group Hamas would move away from militancy towards politics, although he said last week's "unfortunate" Israeli attack on Gaza would mean a delay "until everybody feels they've gotten their own back".