President Bush threatened Syria, Iran on Monday with new diplomatic or economic measures to pressure both countries to stay away from the Iraqi politics.
Bush added that the United States had several ways to retaliate if the two countries failed to heed his warning.
"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.
Bush’s new threat comes nearly a week after he previously demanded Syria and Iran to stop their support to the Iraqi resistance, ahead of next month's elections.
Also last week, Iraq's interim defense minister accused both countries of having ties with Abu Musab al-Zarqawi in Iraq.
Bush told reporters that he has consulted U.S. 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," Bush added.
The U.S. claims that Syria sends military equipment to Iraq and allows insurgents to cross its border into Iraq.
However, Syria has strongly rejected those claims, saying it is doing its best to tighten control at the terrain that defines its border with Iraq.
Last October, administration officials said that they were considering tightening U.S. 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 U.S. exports other than food and medicine.
U.S. and the Iraqi government are trying to curb the Iraqi resistance ahead of the country’s national elections, scheduled for January.
The Iraqi interim government and Washington have frequently accused Iran and the Baathist regime in Syria of supporting violence in Iraq.