Former U.S. lawmaker and Green Party presidential candidate Cynthia McKinney, whose relief boat was seized by an Israeli naval ship Tuesday for the second time in a year, is demanding the immediate release of her and 20 other activists.

McKinney, a longtime supporter of Palestinians, said her Greek-flagged boat, the Spirit of Humanity, was carrying medical supplies, cement, olive trees and children's toys to Gaza when it was boarded by the Israeli navy.

"This is an outrageous violation of international law against us," said McKinney. "Our boat was not in Israeli waters and we were on a human rights mission to the Gaza Strip. President Obama just told Israel to let in humanitarian and reconstruction supplies, and that's exactly what we tried to do. We're asking the international community to demand our release so we can resume our journey."

The Israeli military issued a statement Tuesday saying that the boat had attempted to break a blockade of Gaza and was forced to sail to an Israeli port after ignoring a radio message to stay out of Gaza waters.

The statement said navy personnel boarded the freighter Arion without any shots being fired, and those on board were to be handed over to immigration authorities. Israeli Foreign Ministry spokesman Yigal Palmor said Israel was planning to free the crew and passengers. 

"Nobody wants to keep them here," he said. "They will be released as soon as they are checked."

The humanitarian cargo was also to be trucked into the Gaza Strip after a security check. 

In a statement released by the Green Party, McKinney said she had sent appeals to Obama and the State Department for assurances of protection for the relief mission. She said the boat was sailing in international waters when it was seized.

The White House nor the State Department was immediately available for comment.

This isn't the first time a boat carrying McKinney has clashed with an Israeli navy ship. In December, McKinney was among 16 people aboard a medical supply boat that collided with an Israeli naval ship as it tried to enter coastal waters around Gaza.

At the time, the group claimed the Israeli military fired machine guns into the water in an attempt to the stop the boat's progress. But a spokesman for Israel's foreign ministry said "physical contact" was made only after the aid boat failed to respond to radio contact and he denied any gunfire had occurred.

Israel launched an offensive in December against Gaza in an attempt to halt years of Palestinian rocket attacks on southern Israel. The two sides reached a cease-fire in January.

The Associated Press contributed to this report.