American citizens wearing protective head gear wait to board a US Marines helicopter on the grounds of the US Embassy at the northern edge of the capital Beirut in Lebanon

Israel launched fewer air strikes on Beirut Tuesday as foreign countries ferried their citizens out of Lebanon. But aid agencies and the Lebanese government are warning of a dire humanitarian crisis if the Israeli blockade of Lebanon continues.

There were far fewer air strikes around Beirut on Tuesday than on any day since the Israeli offensive in Lebanon began last week. The United States and several European nations have negotiated safe passage with the Israeli military to relay thousands of their citizens to safety.

The British destroyer HMS Gloucester docked in Beirut and took 180 people on board. The ship's commander said it would travel overnight to Cyprus and offload the passengers there before turning around and heading back to Beirut to pick up more. A second British destroyer will also join the evacuation effort.

In remarks broadcast on British television from the dockside, British Ambassador James Watt rejected criticism that the British response was too slow. "Well firstly, there was question of taking the entire British community on one destroyer. We were always going to do it in batches. We've made that clear in our public messages for days. We will be moving much bigger numbers tomorrow and even bigger numbers the day after that," he said.

The U.S. evacuation plan has come in for even more criticism over its slow pace. The Americans have evacuated a few hundred by helicopters, but mass rescues of the thousands of American citizens in Lebanon have not yet started. U.S. Embassy staff and the State Department in Washington said they will be ramping up the rescue efforts over the coming days, using military and commercial ships.

An unknown number of people, including both foreigners and Lebanese citizens, are still stranded in southern Lebanon, which has come under the most severe attack since the Israeli offensive began. Eight members of a Canadian-Lebanese family were killed in an air strike in the south on Sunday.

The roads leading out of the region have been destroyed, and continued air raids are making it nigh impossible for the United Nations or individual countries to get people out of that region. Ambassador Watt said they are his top priority. "There are others, and those are the ones who are really on my mind, who are stuck in the south of the country, who can't move up to Beirut and we can't go down there easily because of the difficult military situation. We are working on solutions for that. They're the ones we're really concerned for," he said.

In the meantime, the United Nations is bracing for a major humanitarian catastrophe if the Israeli blockade of Lebanon continues much longer. There are already some signs that people are hoarding essential supplies, like drinking water, in expectation of shortages over the coming weeks.

The acting head of the UN refugee agency's Beirut office, Arafat Jamal, said the agency is already planning to confront that situation if it arises. "For us, speaking for my agency, we have a lot of stockpiles for example in Jordan, and a few in Syria, that we are very much thinking of bringing over. These are basically contingency stocks mainly intended for Iraq, we thought there might be an emergency over there, but we will divert them to Lebanon now," he said.

UNHCR is sending an emergency response team to Lebanon to help with the crisis. The U.N. estimates that up to half a million people have been forced from their homes by the air strikes.