The United States expressed concern Tuesday about an unknown number of Americans stranded in south Lebanon without safe passage to evacuation points in the capital and on the coast. "We are aware that there are an undetermined number of Americans at locations in southern Lebanon," the US State Department said in a response report issued Tuesday. Some 15,000 Americans have been evacuated from Lebanon since fighting erupted two weeks ago. The last scheduled boatload of US citizens would leave Beirut Wednesday afternoon, officials said. Israeli airstrikes and sporadic incursions by ground troops have made it unsafe to travel in large areas of the country, especially the south where fighting was most intense. U.S. officials said they could not estimate how many Americans were stranded there. They said they were evaluating ways to rescue any remaining Americans but would not comment on the nature of their plans. "We remain concerned about the welfare of all American citizens in Lebanon. We continue to facilitate their departure and return to the United States," the State Department said. US officials said they had requested space for 300 Americans on a Cypriot ship, the Princesa Marissa, that left Tyre Monday afternoon. It was one of the only vessels to pick up evacuees at the southern port city, rather than Beirut. American officials have been phoning the homes of US citizens and their relatives in south Lebanon, urging them to travel to Tyre. They had hoped some caught the Cypriot ship, but were unsure how many - if any - were able to board before it pulled out of port. The ferry boat arrived in Cyprus early Tuesday, carrying some 230 mostly European nationals. A Canadian ship was due in Tyre on Wednesday to evacuate more people, and Americans would be allowed on board if room was available, officials said.