CAIRO, June 3 (Marwa Awad/Reuters) - Arab states will ask the UN Security Council to demand an end to Israel's blockade of Gaza, the Arab League chief said on Thursday after ministers met to respond to Israel's attack on a Gaza-bound aid flotilla.

Defending Israel's enforcement of its blockade, Prime Minister Benjamin Netanyahu said on Wednesday it was vital for security and would stay in place.

Israeli marines on Monday stormed a Turkish aid ship bound for the Gaza Strip, which is controlled by Hamas Islamists and blockaded by Israel. Nine pro-Palestinian activists were killed in the incident, which sparked widespread condemnation.

The Arab League will go to the Security Council and demand lifting of the blockade from Gaza, the League secretary general, Amr Moussa, told a news conference after the talks.

There must be steps taken to end the siege, Moussa said. We ask all nations to ignore the Gaza blockade and send aid to Gaza to break the blockade.

Turkey, a Muslim country that had been Israel's strategic ally, accused it of state terrorism and has recalled its ambassador from Tel Aviv and demanded it lift its blockade.

Those calls have been echoed by European leaders and the United Nations whose Human Rights Council voted to set up an independent fact-finding mission into the incident.

Israel's key backer, the United States, is less outspoken. It has called for calm. Western powers agree with Israel that Iranian-backed Hamas is a threat but say the embargo should not punish the 1.5 million people trapped in the Gaza Strip.

Egypt opened its Rafah border crossing to Gaza indefinitely on Wednesday -- a step seen as an attempt by Cairo to deflect criticism of its role in imposing the Gaza blockade.

Cairo has kept its border with Gaza largely closed since the Hamas Islamist group, an offshoot of the opposition Egyptian Muslim Brotherhood, seized the territory in 2007.

(Writing by Edmund Blair; Editing by Michael Roddy)