Pro-Palestinian activists say Israeli naval forces are boarding their two ships in international waters, as they were trying to break Israel's blockade of Gaza.

Twitter accounts associated with the activist groups running the flotilla said Friday that Israeli forces were boarding their vessels.

The reports came shortly after an Israeli defense forces spokesperson said on the IDF's official Twitter account that the chief of staff had ordered the navy to board the vessels after all attempts to contact them failed.

The IDF also posted on YouTube a video showing what it says were Israeli naval officers warning the activists' ships that they were attempting to breach a legal blockade. The video also shows the naval officer offering the activists access to Israel for the transportation of their cargo to Gaza.

A spokeswoman for the activists told VOA earlier that the ships are carrying medical supplies and letters of solidarity for the Palestinian people.

Contact with the ships was lost shortly after two Israeli navy vessels intercepted them Friday evening local time.

The 27 activists from countries including the United States, Canada, and Ireland set sail from Turkey on Wednesday. They say their goal is to deliver medical aid directly to Palestinians in Gaza, in defiance of Israel's blockade.

Israel imposed a naval blockade on the Hamas-ruled territory in 2007 in what Israeli officials say is a bid to keep weapons from reaching militants in Gaza.

On Thursday, the U.S. State Department renewed a call for Americans to avoid participating in the flotilla, saying they could face legal consequences. At least two Americans are on board.

Last year, nine activists were killed after Israeli commandos stormed a Turkish-led aid flotilla headed for Gaza. The confrontation caused a deterioration in relations between Turkey and Israel.

In July, pro-Palestinian activists launched an unsuccessful attempt to reach the Palestinian territory.