The United States is sending to Lebanon three C-17 aircraft shipments of relief aid following the massive explosion at Beirut's port that killed at least 137 people, with the first plane filled with food, water and medical supplies arriving Thursday.

U.S. Central Command (CENTCOM) said the first C-17 had delivered 11 pallets of aid from a U.S. military base in Qatar, with the two other plane shipments expected to provide more food and water within 24 hours.

"We are closely coordinating with the Lebanon armed forces and expect that we will continue to provide additional assistance throughout Lebanon's recovery effort," CENTCOM's chief, Marine General Kenneth "Frank" McKenzie, who oversees military operations in the Middle East, said Thursday.

McKenzie spoke with the commander of Lebanon's armed forces, General Joseph Aoun, to express his condolences for the loss of life and devastation caused by the blast, according to the command.

Presumptive Democratic presidential nominee Joe Biden expressed sympathy for the victims of the explosion and urged the Trump administration and the international community "to immediately mobilize assistance to the thousands injured in the blast."

Sending aid to Lebanon is politically tricky because the government is dominated by Hezbollah, which the U.S. and several other nations have designated as a terrorist organization. The Iranian-backed Shi'ite Muslim political party's militant wing has a history of carrying out global terror attacks and is motivated by its opposition to Israel and its resistance to Western influence in the Middle East.

Working with USAID

"We're well aware of some of the concerns with whom the aid would go to, and ensuring that the aid gets to the people of Lebanon that need it most," chief Pentagon spokesman Jonathan Hoffman told reporters Thursday, noting that the military was working with the State Department and the U.S. Agency for International Development on the aid delivery.

A day earlier, U.S. Defense Secretary Mark Esper told the Aspen Security Forum that the U.S. had reached out to the Lebanese government and was positioning its military to assist the Lebanese people.

Esper clarified that there was nothing to suggest the explosion was triggered on purpose.

"Most believe it was an accident, as reported," he said, adding, "It's a shame to see it happen. When you see the video, it's just devastating."

President Donald Trump said Wednesday that he had "heard it both ways" - that it could have been an accident or "a bomb."

The U.S. is willing to help with the investigation into the explosion should the Lebanese government request it, according to Hoffman.

The U.S. State Department said Wednesday that Secretary of State Mike Pompeo had reaffirmed Washington's "steadfast commitment to assist the Lebanese people" to Prime Minister Hassan Diab.

National Security Correspondent Jeff Seldin and State Department Correspondent Nike Ching contributed to this report from Washington.