The United Nations on Friday staged its own tribute to September 11 attack victims with a top UN official calling for new efforts to agree on an international counter-terrorism convention. UN ambassadors staged a minute''s silence in a ceremony two days before the 10th anniversary of the attacks on the United States that killed about 3000 people. The president of the 193-nation General Assembly, Joseph Deiss, called for new work on an anti-terrorism convention which has been under discussion since 1972. One major obstacle has been the failure to agree the definition of a terrorist. But since September 11, the United Nations launched a global strategy on terrorism to increase international cooperation in 2006, while the UN Security Council sanctions committee has played a key role in the battle against terrorism. Despite these important steps to strengthen measures to combat this scourge, the perpetrators of terrorist acts continue to strike," Deiss, a former foreign minister for Switzerland, told the ceremony, highlighting the attack last month on the UN headquarters in Nigeria in which 23 people were killed. "Given the proliferation of terrorist threats and actions, we must intensify our efforts without delay" to agree on a convention, Deiss added. A ministerial counter-terrorism meeting is to be held at the UN General Assembly summit this month and Deiss said: "I call on all member states to use this opportunity to make significant progress on the swift completion of a comprehensive convention on international terrorism." UN chief Ban Ki-moon has earlier renewed its call for a global counter-terrorism treaty.