Iran has warned that it could send Revolutionary Guard naval units to escort humanitarian aid convoys seeking to break the Israeli blockade of Gaza – a move that would certainly be challenged by Israel.

Any such Iranian involvement, raised today by an aide to the supreme leader, Ayatollah Ali Khamenei, would constitute a serious escalation of already high tensions with Israel, which accuses Tehran of seeking to build a nuclear weapon and of backing Hamas, the Islamist movement that controls Gaza.

"Iran's Revolutionary Guard naval forces are prepared to escort the peace and freedom convoys that carry humanitarian assistance for the defenceless and oppressed people of Gaza with all their strength," pledged Hojjatoleslam Ali Shirazi, Khamenei's personal representative to the guards corps.

The threat came as the Israeli prime minister, Binyamin Netanyahu, dismissed a UN proposal for an international commission to investigate last week's commando assault on aid ships, in which nine people died. Another aid ship, the Rachel Corrie, carrying Irish and other peace activists, was boarded peacefully by Israeli forces on Saturday, escorted to the port of Ashdod, and its passengers deported.

Netanyahu has defended Israel's right to maintain the blockade by arguing that without it Gaza would become an "Iranian port" and Hamas missiles would strike Tel Aviv and Jerusalem. Israel's undeclared aim is to weaken or bring down the Hamas government.

Iran continued to exploit the "freedom flotilla" affair to lambast Israel. Its foreign minister, Manuchehr Mottaki, told the Organisation of the Islamic Conference in Jeddah on Sunday that Israel's crime was "another instance of the Zionist regime's brazen and merciless treatment of Muslims, especially the oppressed Palestinian people."

Mottaki also called for a UN resolution condemning Israel. The security council is discussing imposing new sanctions on Iran because of its failure to meet international demands over its nuclear programme.

Iran and Israel have had no diplomatic relations since the 1979 revolution and President Mahmoud Ahmadinejad regularly predicts the disappearance of the Jewish state as well as denying the Holocaust.

Shirazi said Iran should encourage international efforts to break the blockade. "We should expose our enemies to spontaneous global action and not let them achieve their heinous goals," he was quoted as saying by the semi-official Mehr news agency.

Iran's Revolutionary Guards, which have a command structure separate from the regular armed forces, are fiercely loyal to the supreme leader. Khamenei has attacked the raid as a "mistake" that "showed how barbaric the Zionists are".

Israel's determination to strike at links between Iran and Hamas was dramatically demonstrated in January when presumed Mossad agents in Dubai assassinated Mahmoud al-Mabhouh, who was described as the Hamas official in charge of smuggling Iranian weapons into Gaza.

Israel's no-compromise attitude to aid convoys could be tested again after two Lebanese organisations pledged to send boats to Gaza in the next few days. Reporters Without Borders is attempting to assemble 25 European activists and 50 journalists for a boat leaving Beirut. The Free Palestine Movement is planning a similar attempt.

George Galloway, the founder of Viva Palestina, announced in London that two simultaneous convoys "one by land via Egypt and the other by sea" would set out in September to break the Gaza blockade. The sea convoy of up to 60 ships will travel around the Mediterranean gathering ships, cargo and volunteers.