By Humeyra Pamuk

WASHINGTON (Reuters) -U.S. Secretary of State Antony Blinken headed to Israel on Wednesday on a Middle East mission to prevent a wider war from erupting after an attack and hostage-taking by Palestinian Hamas militants and an Israeli bombardment of the Gaza Strip.

In a show of solidarity with Washington's closest Middle East ally, Blinken was due to meet senior Israeli officials, including Prime Minister Benjamin Netanyahu, to discuss further boosting military support.

"I'm going with a very simple and clear message ... that the United States has Israel's back," Blinken told reporters before boarding a plane bound for Israel.

He will work with regional U.S. allies to try to secure the release of more than 100 people that Israel says Hamas holds captive, some of whom may be American citizens. A senior State Department official said Blinken will visit other countries in the region after making stops in Israel and Jordan.

The top U.S. diplomat, Blinken flew out as Israel was forming an emergency unity government. The U.S. State Department did not immediately respond to a request for comment.

Hamas gunmen rampaged through Israeli towns over the weekend, killing 1,200 people and taking scores of hostages to Gaza.

At least 22 Americans were killed during the attack, Blinken said.

"That number could still go up, and it probably will," Blinken said, adding that U.S. officials were working with Israeli counterparts to determine the fates of other U.S. citizens who remained unaccounted for.

Israel has retaliated with air strikes that have killed more than 1,100 people, according to Gaza's Health Ministry. The Israeli military said its troops had killed at least 1,000 Palestinian gunmen who infiltrated from Gaza.

Asked if Washington had advocated for Israel to exercise restraint in its response, with a ground operation in Gaza expected, Blinken said that Israel respects international law and makes efforts to avoid civilian casualties.

"We know that Israel will take all of the precautions that it can, just as we would, and again that’s what separates us from Hamas and terrorist groups that engage in the most heinous kind of activities," Blinken said.

A top priority for Blinken will be to convey a message of deterrence, largely aimed at Iran and Iran-backed groups such as Lebanon's Hezbollah, to stop a wider war from erupting.

Hezbollah has moved carefully since Hamas and Israel went to war, keeping Israeli troops busy with attacks at the Lebanese border but not opening a big front, sources familiar with its thinking say.

Violence on the border between Israel and Lebanon flared up for a fourth day on Wednesday with Israeli shelling hitting southern Lebanese towns in response to a fresh rocket attack by Iran-backed Hezbollah.

NO WEST BANK VISIT IN BLINKEN TRIP ANNOUNCEMENT

Since Saturday, Blinken has spoken on the phone with his counterparts from Egypt, Jordan, Saudi Arabia, Qatar, Turkey and the United Arab Emirates. U.S. officials said Washington was pushing regional countries with influence on Hamas and others hostile to Israel to help stop the conflict from worsening.

"The attack by Hamas is a reminder of the perception of an American absence or lack of commitment to the region that some actors might interpret and do things they should not be doing," Alex Vatanka, director of the Iran program at the Middle East Institute said.

Biden on Wednesday called the Hamas attack "sheer evil", reaffirmed U.S. support for Israel and issued an apparent warning to Iran and Iran-backed groups against exploiting the conflict: "I have one word: Don't."

Blinken's trip announcement did not include the Israeli-occupied West Bank. On previous trips to the region, Blinken has visited the West Bank, controlled by the Palestinian Authority and its President Mahmoud Abbas.

Israel has also tightened its siege in Gaza, which it has kept under a blockade since Hamas seized power there in 2007, saying it will keep out food and fuel while vowing to further escalate with a ground offensive.

Washington said it was talking with Israel and Egypt about the idea of safe passage for civilians from Gaza, another key topic that Blinken may be discussing with his counterparts during the trip.

Biden has stopped short of an overt plea to Israel to show restraint to avoid Palestinian civilian casualties in Gaza. It was unclear whether Blinken might make such an appeal when he meets Israeli officials behind closed doors.

(Reporting by Humeyra Pamuk, additional reporting by Matt Spetalnick and Simon Lewis; Editing by Michelle Nichols, Howard Goller and Lincoln Feast.)