The Obama administration is stepping up pressure on activists planning to challenge Israel's sea blockade of the Gaza Strip, warning that they will face action from Israeli authorities and that American participants may also be violating U.S. law.

The U.S. State Department said Friday that attempts to break the blockade are "irresponsible and provocative" and that Israel has well-established means of delivering assistance to the Palestinian residents of Gaza. It noted that the territory is run by the militant Hamas group, a U.S. designated foreign terrorist organization, and that Americans providing support to it are subject to fines and jail.

Open gallery view Israel Navy forces approaching the Mavi Marmara bound for Gaza, May 31, 2010. Credit: Reuters

"Groups that seek to break Israels maritime blockade of Gaza are taking irresponsible and provocative actions that risk the safety of their passengers. Established and efficient mechanisms exist to transfer humanitarian assistance to Gaza. For example, humanitarian assistance can be delivered at the Israeli port of Ashdod, where cargo can be offloaded, inspected, and transported to Gaza," a State Department press release said.

"We urge all those seeking to provide such assistance to the people of Gaza to use these mechanisms, and not to participate in actions like the planned flotilla."

The warning is the third in as many days and follows the announcement by 36 Americans that they will sail aboard a U.S.-flagged vessel in a flotilla to Gaza.

The statement also reiterated the U.S. stance on Hamas, calling the Islamic group "to play a constructive role by renouncing violence, recognizing Israels right to exist, and accepting past agreements."

Despite warnings by the State Department, one traveler planning to ignore that advice is celebrated poet and novelist Alice Walker, who intends to join with other European and American activists when they set sail from the ports of some 22 countries.

"Why am I going on the Freedom Flotilla II to Gaza? I ask myself this, even though the answer is: What else would I do?," Walker wrote, explaining her motives in an open letter to CNN and outlining her plan to carry letters to the people of Gaza on board the Audacity of Hope boat. "We will be carrying letters ... expressing solidarity and love," she writes.

Walker's letter goes on to discuss the brave "followers of Gandhi," and the "Jewish civil rights activists" who stood side by side with blacks in the South, and places her current "mission" within this context. She ends with a rebuke to both Prime Minister Benjamin Netanyahu's policies and those in the U.S. that back them.

Meanwhile, the Turkish humanitarian relief foundation IHH has announced that the refurbished Mavi Maramara ship, on which nine activists were killed during an attempt to break Israel's coastal siege on the Gaza Strip in May 2010, will not join the flotilla planned to set sail at the end of the month.