KAMPALA, June 1 (Frank Nyakairu and Aaron Gray-Block - Reuters) - UN Secretary-General Ban Ki-moon urged Israel on Tuesday to lift its Gaza blockade, saying that if Israel had heeded international calls to do so the raid on a Gaza aid ship flotilla would not have happened.

Had the Israeli government heeded international calls and my own strong and urgent and persistent call to lift the blockade of Gaza, this would not have happened, Ban told Reuters in an interview on Tuesday in Kampala.

Israel detained or deported on Tuesday hundreds of activists who were on the Turkish-backed aid ships it seized on Monday, while the UN Security Council called for a prompt and credible investigation into the raid, which led at least nine deaths.

Ban said another, most depressing issue was the need for the release by Israel of the detainees and the delivery of the humanitarian aid to Gaza, stressing he was in talks about the issue with the Jewish state and UN staff on the ground.

Israel imposed the blockade to try to undermine the Palestinian Islamist movement Hamas, which seized control of the sliver of desert territory on the Mediterranean coast in 2007.

Attending a Kampala review conference of the International Criminal Court, the world's first permanent war crimes court, Ban also praised the work of The Hague-based body.

He said no one would have expected the ICC, when it was first established, to have established itself so vigorously as a permanent court now trying and prosecuting perpetrators.

ROBUST PEACEKEEPING IN CONGO
Ban also rejected criticism about continuing violence in Africa, such as in the Democratic Republic of Congo, and said robust and effective peacekeeping operations had been deployed.

But considering the size of the territory and complexity of political backgrounds of all these countries, sometimes there is a limit to cover a whole area of the African continent, he said, adding peacekeeping operations require state support.

The Security Council on Friday had authorised the withdrawal of up to 2,000 troops from the UN peacekeeping force in the Congo over the next month, but made no pledge on further cuts despite pressure from Kinshasa to withdraw the troops.

Potential investors and human rights groups fear a too hasty withdrawal of the 20,500-member MONUC force would trigger more violence in a country still struggling to recover from a 1998-2003 war and battling rebels across its territory.

Also addressing further problems in Somalia -- plagued by violence and instability -- Ban called for the strengthening of the Somali armed forces, police and peacekeeping missions.

But he also said the international community needed to support the transitional government in its socio-economic development efforts, which could in turn help curtail continued piracy in the Gulf of Aden off the coast of Somalia.

There are so many young people who don't have any job opportunities. Therefore, we need give some incentives where people can engage in social economic activities, he said.

Ban will present a report to the UN Security Council by the end of July, discussing issues such as prosecuting pirates.