The UN's top Mideast envoy warned Wednesday that Gaza could erupt in renewed and more devastating violence unless there is a lasting cease-fire. Since unilateral cease-fires were declared by Israel and Hamas a month ago, Robert Serry said Palestinian militants in Gaza have fired 15 rockets and 12 mortars toward Israel and detonated an Israeli jeep near the border fence - "attacks that are as irresponsible as they are unacceptable" - while Israel has launched 19 airstrikes in Gaza. "There is an urgent need for all acts of violence to cease and for full respect of international humanitarian law by all parties," he said. Serry spoke to the UN Security Council shortly before Israel announced that it will not open Gaza's blockaded borders until Hamas terrorists free captured IDF soldier Gilad Schalit. The decision was a major blow to Egyptian efforts to broker a long-term cease-fire between the two enemies, and was immediately condemned by Hamas, which desperately wants the blockade lifted so it can start repairing the widespread damage from Israel's military offensive in Gaza last month.