JERUSALEM, Dec 22 (Douglas Hamilton/Reuters) - The United Nations on Wednesday sharply condemned a rise in cross-border attacks by Palestinian militants in Gaza, a day after a rocket exploded close to an Israeli kindergarten.

The U.N. 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 militant Islamist 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 U.N. envoy noted the Israeli air strikes, saying Israel had a right to self-defence 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 militant 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 relevent United Nations resolutions, and not use a policy of double standards, he said.

RESTRAINT

Military analysts believe a second major Gaza offensive by Israel is not imminent, though it may be inevitable in the long run. They said Tuesday's rocket attack, which caused no injuries in the kindergarten, could have triggered a powerful Israeli response had children been hurt or killed.

Deputy Prime Minister Dan Meridor, speaking on Israel Radio, said: We have no interest in a development of hostilities. And if the other side will maintain total quiet there is no reason that such actions will develop.

Hamas, he said, has not done enough to stop rockets.

Serry said daily life for Gaza's 1.5 million Palestinians had improved following Israel's relaxation of restrictions on imports and work to expand trade potential at a logistics hub.

Calm was essential to the success of the measures, he said.

More than 200 rockets and mortars have been fired from Gaza into southern Israel this year. Only one attack was fatal, when a Thai farm labourer was killed by a mortar in March.

Israel's air strikes targeting armed militants and rocket squads inside Gaza are often lethal. (Additional reporting by Nidal al-Mughrabi in Gaza, Ali Sawafta in Ramallah and Jeffrey Heller in Jerusalem; Editing by Peter Graff)