Israeli warships turned back a Greek-flagged vessel carrying aid for Palestinians in Gaza, activists said on Thursday.
Activists on board the ferry said they were returning to Cyprus after an overnight encounter in international waters in which, they said, Israeli ships threatened to open fire.
The boat carrying 21 people was intercepted about 100 miles northeast of Gaza, said Huwaida Arraf, an organiser of the mission by the U.S. based Free Gaza Movement.
"They got very close and they threatened that if we continued they would open fire on us," Arraf told Reuters from the vessel, now sailing back to Cyprus.
"They surrounded us with about four warships making it very difficult to navigate. They said they would use all means to keep us out of Gaza," she said.
Spokeswoman Karin Pally said the people on the boat decided to head back.
On Wednesday Israel ordered an Iranian vessel carrying humanitarian aid for Palestinians to turn back.
Both vessels were planning to sail directly to Gaza port, rather than the checkpoints controlled by Israel and Egypt on either end of Gaza. The boat from Cyprus included politicians, aid workers and doctors.
An Israeli navy blockade on Gaza has intensified since it launched an assault on Dec. 27 to rout Hamas militants from the enclave.
The Free Gaza Movement started sending regular shuttles of aid to Gaza from Cyprus in August 2008.