"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 news conference.
The president is reviewing a wide range of options, including freezing the assets of high-ranking Syrian government officials, US officials said.
Bush said he had discussed with American generals "whether or not there are former Saddam loyalists in Syria ... funnelling money to the insurgents".
"We ought to be working with the Syrian government to prevent them from either sending money and/or support of any kind," he added.
Washington accused Syria of sending military equipment to Iraq during the US-led invasion last year.
Since then it has claimed that Damascus lets fighters cross its border into Iraq and harbours former officials directing the fighters.
Bush blames Syria and Iran for
the unrest in Iraq
Damascus denies those charges and says it is doing its best to tighten control of the hundreds of miles of mostly desert terrain that define its border with Iraq.
Administration officials said in October they were considering tightening US economic sanctions on Syria to put pressure on Damascus to pull its troops out of Lebanon, and crack down on what it calls terrorism.
In May, Bush imposed a series of sanctions on Syria, including a ban on US exports other than food and medicine.
He also accused Damascus of pursuing weapons of mass destruction (WMD) and failing to stop anti-US fighters from entering Iraq.
Bush's threat comes less than a week after he demanded that Syria and Iran stop fighters and money from entering Iraq before next month's elections.
"When I said the other day that I expect these countries to honour the political process in Iraq without meddling, I meant it. And hopefully those governments heard what I said," he added.
The US-selected interim Iraqi government and Washington frequently blame long-time foe Iran and the Baathist government in Syria for supporting the unrest in Iraq.