(Reuters) - Islamist gunmen killed at least 15 Egyptian police on Sunday and seized two military vehicles to attack a crossing point into Israel, the deadliest incident in Egypt´s tense Sinai border region in decades.

Israeli aircraft destroyed one of the armored vehicles and four of the gunmen were killed, Egyptian security sources said. Israel´s Defense Minister Ehud Barak called for "determined Egyptian action" to "prevent terror in Sinai".

The attack is an early diplomatic test for Egyptian President Mohamed Mursi, an Islamist who assumed office at the end of June after staunch U.S. ally Hosni Mubarak was overthrown last year in a popular uprising.

It may also complicate Egypt´s relations with Hamas, the Islamist rulers of the Gaza Strip that borders Egypt and Israel who are close to Mursi´s Muslim Brotherhood, if it is shown that Palestinian gunmen were involved.

Israel has accused Palestinian militants in Gaza of involvement in militant activity in Sinai, where insecurity has grown since Mubarak was toppled.

Mursi has pledged to get a grip on security in Egypt but has yet to prove he can assert control over an entrenched security establishment.

Mursi summoned senior military officials and vowed to respond to the attack. His spokesman said the assailants had not been identified for now.

"Clear orders have been given to our armed forces and police to chase and arrest those who carried out this assault on our children," Mursi said in a speech on state television. "The forces will impose full control over these areas of Sinai."

The Egyptian state news agency quoted a security official as saying the attack was carried out by Islamist militants who infiltrated from Gaza via a tunnel as well as by other militants from two regions in Egypt´s Sinai peninsula.

Hamas said it was investigating the incident and had no information so far indicating the gunmen came from Gaza but Taher al-Nono, a spokesman for the Hamas government, said it was closing tunnels along the border with Egypt immediately.

Israeli Prime Minister Benjamin Netanyahu said the Israel Defense Forces and members of the internal security agency had foiled a big attack against Israeli civilians.

"Whoever intends to attack us must know that our long arm will find him," he said in a statement via Twitter.

DUSK AMBUSH

The gunmen ambushed the Egyptian border police who had gathered to break the Muslim Ramadan fast after sunset.

"Elements of a jihad organization drove SUVs into one of the checkpoints south of the Ramah border," Egyptian state television said, referring to the main Gaza-Egypt crossing point. It said 15 security forces personnel were killed and at least 7 wounded.

One of the stolen army vehicles exploded as the militants tried to storm the Israeli border and the second was targeted by the Israeli air force at the Kermes Shalom crossing into southern Gaza, a military zone where the Israeli, Egyptian and Gaza borders intersect, a spokesman for Netanyahu said.

It was not clear how many attackers were involved.

"As of now, we know of no terrorists still in the area. There are some Israeli communities nearby, and residents have been asked to stay in their homes," Israeli military spokeswoman Aviate Leibovich said.

An Egyptian security source said the Ramah crossing had been closed "indefinitely" after the attack.

FORTIFIED BORDER TRIANGLE

Netanyahu said in April that Sinai was becoming a lawless "Wild West" but his government has since signaled more confidence in Egypt´s ability to restore order.

Jihadi groups have hidden out in the lawless Sinai for almost a decade but security forces hemmed them in under Mubarak.

They became more active after Egypt´s uprising of January 2011 with attacks on security posts, military roadbocks and a key gas export pipeline and occasional raids on the heavily defended border with Israel.

The militants, who are mostly members of Bedouin tribes who long complained of neglect by the government in Cairo, say they want to fight Israel to free occupied lands and have links with some jihadi groups in Gaza, according to Egyptian security experts.

Earlier on Sunday, an Israeli air strike killed a Palestinian gunman from a radical Islamist group and wounded another as they rode a motorbike in southern Gaza near the Egyptian border.

It was not immediately possible to confirm whether the incidents were linked.

Hamas, while hostile to Israel, is considered too moderate by many Salafis and has at times clashed with them during law and order drives.

Mursi welcomed Hamas leaders to Cairo last month after he took office but is also keen to show Israel´s main ally the United States that Egypt can be a reliable partner that respects its international treaties including its peace deal with Israel.

(Additional reporting by Yusri Mohamed in Sinai and Ari Rabinovitch in Jerusalem and Tamim Elyan and Ahmed Tolba in Cairo; Writing by Douglas Hamilton and Tom Pfeiffer; Editing by Andrew Osborn) (© Thomson Reuters 2012. 08/05/12)