Sen. John McCain (R-Ariz.) answers journalists' questions during a news conference at the David Citadel hotel in Jerusalem on Jan. 3. (POOL/REUTERS)

Leading Republican senators said Friday that they share Israeli Prime Minister Benjamin Netanyahu’s reservations about terms for an Israeli-Palestinian peace agreement proposed by Secretary of State John F. Kerry.

Netanyahu “has serious, serious concerns about the plan as it has been presented to him,” particularly about the future security of Israel and the viability of a future independent Palestinian state, Sen. John McCain (Ariz.) said after meetings with Netanyahu and other officials in Israel.

McCain and fellow Senate Republicans Lindsey O. Graham (S.C.) and John Barrasso (Wyo.) said they share the general alarm about Israel’s security should the country withdraw from the West Bank. Without providing details of the largely secret proposals, McCain and Graham suggested that they and other supporters of Israel in Congress will greet Kerry’s program skeptically.

“We feel very strongly that the peace process is very important sooner or later, and we support the legitimate peace process,” McCain said. But he expressed concern about whether some aspects of the agreement are “truly enforceable and viable options” that would not put Israel in jeopardy.

McCain and Graham met with Kerry on Friday in Jerusalem, where the secretary of state is working to win Israeli and Palestinian backing for a rough outline of a peace deal. Kerry met with Netanyahu on Friday for the second time in two days, and he met later with Palestinian Authority President Mahmoud Abbas.

Late Thursday, Israeli security forces announced the arrest of four operatives and 10 accomplices they said belong to an alleged terrorist cell based in Bethlehem. The Israelis said that during interrogations the suspects had confessed to a remote-controlled bombing on an Israeli bus in a Tel Aviv suburb Dec. 22.

The bus bombing and subsequent arrests highlight a less obvious but potentially significant challenge for Kerry separate from winning agreement from Netanyahu and Abbas. Other players who are not at the negotiating table but want to influence the outcome, including Iranian-backed and other terrorist cells in the Gaza Strip and the West Bank, oppose any peace deal between Israel and the Palestinian Authority and are prepared to wage a campaign of violence to stop it.

Details of what Kerry calls a framework agreement ideally would be worked out before an unofficial deadline he set for a deal by the end of April. Kerry’s focus on securing an interim agreement, however, is widely seen in Israel and among Palestinians as a sign that talks could extend past then.

McCain, speaking for the Republican group, said Kerry’s energetic push for a peace deal is welcome, and he was careful not to criticize Kerry or give any point-by-point evaluation of his efforts. The three senators’ trip to Afghanistan and the Middle East was scheduled before Kerry announced his latest Jerusalem-West Bank shuttle diplomacy.

But McCain said the Obama administration is allowing Syria to collapse and potentially endangering Israel through generous terms for nuclear rapprochement with Iran.

Graham said that despite detailed security proposals for the West Bank developed by a special U.S. envoy, retired Marine Gen. John R. Allen, senior Israelis remain unconvinced. “Here’s the one thing that I think dominates the thinking in Israel: that once you withdraw, then the ability to go back is almost impossible,” Graham said. “Look at Gaza. What’s the chance of going back into Gaza militarily?”

Israel can defend itself against rocket attacks from that formerly Israeli-occupied territory, but withdrawal meant giving up the “ability to chart your own destiny,” Graham said.

“I really do believe that the idea of withdrawing has to be considered in light of Gaza,” Graham said.

Israeli air force planes fired missiles into the Gaza Strip on Friday in a response to rocket fire from Gaza into Israel, the first such exchange of 2014. Last year was one of the quietest in a decade of confrontations between Israel and the coastal enclave, from which Israel unilaterally withdrew in 2005. Gaza is governed by the militant Islamist group Hamas, which has close ties to the Muslim Brotherhood in Egypt.

Israeli military spokesmen said the warplanes targeted “a terror infrastructure site” and three concealed rocket launchers in Gaza. Reporters in Gaza said the Israeli missiles landed in agricultural fields.

No injuries were reported.

In the bus bombing last month, a passenger saw a suspicious bag left behind, and when he opened it, he saw wires. He alerted the driver, who ordered everyone off the bus. The bomb exploded several minutes later.

The Israel Defense Forces said the device had been prepared by Shadi and Hamdi Ta’amri, two of the four whose arrests were disclosed Thursday and who had previously been arrested for alleged connections to the group known as the Islamic Jihad Movement in Palestine.

The incident stirred memories among Israelis of the second Palestinian intifada, or uprising, a decade ago, a time that saw weekly assaults, including devastating bus bombings and suicide attacks, as well as an Israeli counteroffensive of arrests, curfews and operations to thwart it.

Israeli authorities said the bomb — made of improvised explosive material and packed with nails and screws — was hidden in a bag and given to Sami Harimi, also arrested, who sneaked into Israel from the occupied West Bank through a breach in the separation barrier.

Jonathan Fighel, a retired Israeli intelligence officer and a senior researcher at the International Institute for Counter-Terrorism here, said members of the Palestinian Islamic Jihad “oppose any deal between Israel and the Palestinian Authority.”

They were driven to attack, Fighel suggested, to thwart the peace process, especially the impending release of a third group of 26 Palestinian prisoners serving long sentences.

The Palestinian Islamic Jihad movement is “completely rejectionist,” Fighel said. “They are party spoilers, and they want to spoil this party.”

Ruth Eglash contributed to this report.