The UN Special Coordinator for the Middle East Peace Process, Robert Serry, said rocket strikes from Gaza at Israel, which had escalated in recent days, were "in clear violation of international humanitarian law" and endangered civilians.

The criticism drew a strong response from Hamas, the group which controls the enclave under Israeli blockade, which said Serry's remarks reflected "double standards."

In two days this week at least 14 rockets and mortars were fired at southern Israeli territory.

Israel has launched air strikes in response, including one which killed five Palestinian militants at the weekend, the highest single toll since a three-week Israel offensive in Gaza two years ago in which 1,400 Palestinians and 13 Israelis died.

The UN envoy noted the Israeli air strikes, saying Israel had "a right to self-defense consistent with international humanitarian law." He urged maximum restraint and "every precaution to ensure Israeli forces do not endanger civilians in Gaza."

Israel says Hamas bears responsibility for the increase in missile strikes and has not done enough to stop smaller groups firing across the frontier.

Hamas says Israel is the principle aggressor. Hamas government spokesman Taher Al-Nono said the United Nations should "correct the position expressed by Serry" who was justifying "the aggressive actions of the Israeli occupation."

"The United Nations is required to ... respect the rights of the Palestinian people as stated in international law and in the relevant United Nations resolutions, and not use a policy of double standards," he said.