WASHINGTON (AP) — The number of U.S. troops helping the relief effort in the typhoon-hit Philippines could triple to more than 1,000 by the end of the week, officials said Wednesday.

Senior Obama administration officials said that after a very difficult first few days, they are cautiously optimistic that logistics-caused delays of large quantities of aid materials are easing.

Thousands were killed and 600,000 people displaced by last week's powerful storm that struck the central Philippines, and many remain hungry, thirsty and sick. The Southeast Asian nation is a U.S. treaty ally, and Washington has provided $20 million in immediate aid.

President Barack Obama urged U.S. citizens, too, to contribute, directing them to a White House website with links to groups providing aid. He noted some of the areas hit by the storm are the same places where U.S. and Philippine forces worked together to liberate the islands during World War II.

"Recovering from one of the strongest storms ever recorded will take years," Obama said in a statement. "But the strength, resilience and faith of the Filipino people are legendary."

Coordination at the airport in the hard-hit Tacloban has improved, and a road to the city that was cut by the storm has opened up, which should accelerate the distribution of relief supplies, said administration officials, who briefed reporters about the U.S. response to the disaster. They demanded anonymity under ground rules set by the administration.

"For the first few days ... it was a lot like trying to squeeze an orange through a straw. We are now getting more and bigger straws," said one U.S. official.

The first airlift of hygiene kits and plastic sheeting from the U.S. Agency for International Development was distributed Wednesday to help 10,000 families, and another consignment was due to arrive in the capital, Manila, Thursday.

A United Nations spokesman, Martin Nesirky, said the U.N. World Food Program distributed rice and other items to nearly 50,000 people in the Tacloban area Wednesday.

The U.S. is using C-130 transport planes and Osprey helicopters to transport aid. The transport planes have evacuated about 800 victims of the disaster from Tacloban to Manila. Unmanned aircraft deployed from Guam have provided overhead reconnaissance to help in damage assessment.

U.S. ships are heading toward the area to expand search-and-rescue operations, provide medical care and a platform for helicopters to move supplies to remote areas, the White House said. A carrier and three escort ships are expected to arrive off the coast of the Philippines on Thursday evening, and several U.S. warships and surveillance aircraft are already there.

The U.S. Pacific Fleet commander, Adm. Harry Harris, ordered the activation of the USNS Mercy hospital ship so that it can begin preparations to go to the Philippines if ordered. The USNS Mercy is in San Diego and could be underway in the next several days, but it would not reach the Philippines until sometime in December.

The U.S. military is also helping transport Philippine security forces to enforce a curfew and restore order to the typhoon-hit region, where violence and armed looting has occurred because of lack of basic supplies. The U.S. officials said maintenance of security is the responsibility of the Philippine authorities, and the situation is improving.