ATHENS, August 23, 2008 (AFP) - A group of pro-Palestinian activists sailing for the Gaza Strip to break a year-long blockade accuse Israel of jamming their communications as they neared the area, a Greek left-wing party said on Sunday.

Two converted fishing boats with 44 activists on board expected to reach Gaza on Sunday are unable to use their electronic navigation instruments and transmit distress messages, the Syriza party said in a statement.

'This interference runs contrary to international shipping rules and safe navigation rules,' said Syriza, one of whose parliamentary deputies, Tassos Kourakis, is among the activists.

In an incomplete message transmitted 70 miles from Gaza on Saturday, the boats said they were experiencing 'Israeli electronic piracy,' Syriza said.

Israel has warned the activists to steer clear of the territory, which it says 'is the subject of an (Israeli Navy) advisory notice' that warns off foreign vessels from the 'designated maritime zone.'

The two boats -- Liberty and Free Gaza -- are sailing under Greek flags and are carrying 200 hearing aids for Gaza children and 5,000 balloons.

Many of those making the trip said they had received death threats.

The activists, mostly American and British, include Lauren Booth, sister-in-law of former British prime minister Tony Blair who is now an international Middle East peace envoy.

Ruled since June 2007 by Hamas, an Islamist movement that is considered a terrorist organisation by the West, the Gaza Strip has been under Israeli blockade for the past year except for humanitarian aid.

But the California-based Free Gaza Movement says Israel's aid supply record is 'deplorable'.