The United States had several ways to retaliate if the two countries failed to heed his warning, Aljazeera quoted Bush as saying.

"We have sent messages to the Syrians in the past and we will continue to do so. We have tools at our disposal -- a variety of tools, ranging from diplomatic tools to economic pressure.

"Nothing's taken off the table," Bush told a press conference.

Last week, Iraq's interim defense minister accused both countries of having ties with Abu Musab al-Zarqawi in Iraq.

On Monday, Iraq's ambassador to Iran, Mohammad Majid Al-Sheikh, announced that Iraq's embassy has stopped giving visas to any Iranian national.

"We will only provide those Iranians with visas who hold a permit from the Iranian Foreign Ministry or those Iranian traders who are members of the country's Commerce Chamber," Al-Sheikh was quoted as saying.

Bush's new threat comes nearly a week after he  demanded Syria and Iran to stop their support to the Iraqi resistance, ahead of next month's elections.

Bush told reporters that he has consulted US military officials about whether there are still former Saddam loyalists in Syria financially supporting the resistance in Iraq.

"When I said the other day that I expect these countries to honor the political process in Iraq without meddling, I meant it. And hopefully those governments heard what I said," Bush said.

"We ought to be working with the Syrian government to prevent them from either sending money and/or support of any kind" to the insurgents in Iraq."

Syria has rejected US claims that it was sending military equipment to Iraq and allowing insurgents to cross its border into Iraq.

In October, US administration officials said they were considering tightening economic sanctions on Syria to pressure Damascus to withdraw its troops from Lebanon and crack down on "terrorism".

In May, Bush imposed economic sanctions on Syria, including a ban on US exports other than food and medicine.

The US and the interim government in Iraq are trying to check the Iraqi resistance ahead of the country's national elections scheduled for January. Both have repeatedly accused Iran and the Baathist regime in Syria of supporting violence in Iraq