The U.S. Embassy has asked Americans in Lebanon to be extremely vigilant and avoid non-essential travel because of escalating violence in the conflict with Israel.

A warden message posted on the embassy's Web site said: "We urge Americans residing in Lebanon to maintain a high level of vigilance and to take appropriate steps to increase their security awareness."

The warning comes amid a widening Israeli military offensive that brought to 51 the number of Lebanese civilians killed since Wednesday, when Israel began attacking Lebanon to rescue two Israeli soldiers captured by Hezbollah guerrillas.

Israel has hit hundreds of targets in Lebanon, closing down the country's sole international airport and damaging the main highway to neighboring Syria. Israeli ships are also blockading the coast.

Hezbollah has fired dozens of rockets and mortars into northern Israel in response.

In its Web site posting late Thursday, the U.S. Embassy said it continues to operate normally, but said that all non-essential travel by American officials from the embassy compound had been restricted.

Asked about news reports that the U.S. was bracing for the possibility of evacuating its citizens in Lebanon, a U.S. Embassy spokeswoman said the embassy's "first concern must be for the safety and well being of its citizens."

"We are monitoring the situation closely," she told The Associated Press.