President Bush said Thursday that the United States would freeze the property and assets of anyone trying to undermine Lebanon’s democratically elected government — a move intended as a sharp warning to Syria and its ally Hezbollah.

The announcement, in an executive order and an accompanying letter to Congress, reflects heightened concern in Washington that Syria is trying to reassert control over Lebanon. It comes a little more than a month after the administration announced that it was enacting a travel ban, barring “those who have contributed to the breakdown of the rule of law in Lebanon,” possibly including leading Syrian intelligence officials, from entering the United States.

Taken together, the steps are an effort to ratchet up pressure on Syria at a time when the administration contends that it is helping to fuel the insurgency in Iraq, as well as creating instability in Lebanon. Mr. Bush’s order deems interference in Lebanon’s government to be an “extraordinary threat to the national security and foreign policy of the United States,” and declares it a “national emergency.”

Administration officials say they are especially concerned that the fragile democratic government in Lebanon, headed by Prime Minister Fouad Siniora, could splinter if the Lebanese president, Émile Lahoud, who has close ties to Syria, tries to establish an alternate government. That concern has grown in recent months, said an administration official involved in formulating the executive order.

“We think measures like this visa ban and the executive order not only allow us to target people who are behind this and complicit in it, but they raise the cost of going along with it,” said the official, who did not have authorization to speak on the record. “There are key figures in Lebanon who could go one way or the other, who are sitting on the fence, and this is a way to try to deter them.”

The White House, which has long regarded Syria as a state sponsor of terrorism, is particularly incensed over Syrian support for Hezbollah, the militant group in Lebanon that waged war against Israel last summer. In the months since, Lebanese cabinet members aligned with Syria have resigned from the government, and the United Nations Security Council has voted to form a special tribunal to try Syrians suspected in the 2005 assassination of Rafik Hariri, the former Lebanese prime minister.

Mr. Hariri, who was killed in a car bombing, had opposed Syria’s strong political and military presence in Lebanon. Protests after his death led to the withdrawal of Syrian troops, who had been in Lebanon since the 1970s.

The White House would not identify particular targets of the executive order, but they are likely to include those who were potentially under the travel ban. Among them was Gen. Asef Shawkat, a leading Syrian intelligence official and the brother-in-law of President Bashar al-Assad of Syria. General Shawkat has been implicated in the murder of Mr. Hariri.

Experts say it is unclear what effect, if any, freezing property and assets will have. Jon B. Alterman, a Middle East expert at the Center for Strategic and International Studies in Washington who visited President Assad about a month ago, said Mr. Assad and his associates were deeply concerned that the Siniora government could threaten his own.

“They are concerned that a hostile government in Lebanon will be a means to undermine the government of Syria, and they’re determined to undermine any government in Lebanon that they see as hostile,” Mr. Alterman said, adding, “I don’t think economic measures can deter people from what they regard as their strategic goals.”

New York Times