Israeli Prime Minister Benjamin Netanyahu`s office quoted him as telling U.N. Secretary-General Ban Ki-moon that a mission of ships expected in late May "was being organized by Islamic extremist elements, among others, with the goal of creating a provocation and bringing about a flare-up."
U.N. spokesman Farhan Haq issued a statement saying Ban "noted Prime Minister Netanyahu`s concerns about possible flotilla shipments for Gaza."
"The Secretary-General reiterated that there are land routes available for those wishing to send humanitarian assistance to Gaza," Haq said. "He stressed as well that Israel should take meaningful steps to end the closure of Gaza."
The Free Gaza Movement, a pro-Palestinian activist umbrella group, said the May flotilla would comprise 15 ships with international passengers including Europeans and Americans.
"We sail not just for Gaza," the group said in a March 31 posting on its website. "We sail to confront an entire apartheid regime that must be dismantled through citizen action."
Citing security needs, the Israeli navy stopped a six-vessel flotilla in international waters on May 31, 2010, as it tried to reach Gaza, which is ruled by the Hamas Islamist group.
Marines killed nine Turkish activists in melees aboard the lead ship, fraying Israel`s once-strong ties to Ankara.
Israel has since eased commercial traffic over its land crossings with Gaza, many of whose 1.5 million Palestinians are aid-dependant, but still keeps close control of the sea access.
Regarding the new sailing, Netanyahu said Israel was "committed to acting firmly against the flotilla," according to the Israeli statement. It did not say what the prime minister expected the United Nations to do.
Israel last month caught what it called an Iranian-sponsored shipment of advanced missiles intended for Gaza. Iran denied it.
The United Nations has welcomed measures to relax some trade barriers with Gaza, but wants the blockade, supported by neighbouring Egypt, to be removed entirely.
Gaza was devastated by a 2008-2009 war between Israel and Hamas, which rejects Palestinian peacemaking with the Jewish state. The territory has since seen spikes in violence. Last month Hamas and other factions fired dozens of rockets at Israel and Israeli forces killed 14 Palestinians.
The Free Gaza Movement has called the blockade illegal and flagged persistent Palestinian poverty in the territory. It has championed non-violent political action in a comprehensive challenge to Israel.