U.S. involvement in a coordinated military attack on Iran-allied rebels in Yemen will have "no impact" on nuclear talks with Tehran, a senior U.S. official told AFP on Thursday.

"We have always been clear that the P5+1 negotiations are solely focused on the nuclear issue," the official said, requesting anonymity and referring to the group of six nations engaged in talks.

Iraq's Shiite militias have withdrawn from the forefront of an offensive to recapture Tikrit from Islamic State jihadists, allowing Baghdad government troops to take the lead, a top U.S. general said Thursday.

The Shiite militias "have pulled back from that area" and Iraqi special forces and police are clearing Tikrit, General Lloyd Austin, head of U.S. Central Command, told lawmakers.

U.S. President Barack Obama will host Pope Francis at the White House on September 23, accepting an invitation extended during talks at the Vatican last year.

The White House said the president and the pope will continue a dialogue about poverty, the environment, immigration and promoting religious freedom.

Iraqi forces buoyed by the first U.S.-led coalition air strikes on Tikrit made a final push Thursday to flush diehard jihadists out of Saddam Hussein's hometown.

U.S.-led warplanes carried out 17 air strikes against Islamic State jihadists in Tikrit overnight in the first wave of coalition bombing raids in support of an Iraqi government offensive to recapture the town, the U.S. military said Thursday.

U.S. Secretary of State John Kerry Thursday held a conference call with Gulf ministers to discuss the Yemen crisis amid Saudi air strikes on Huthi rebels, a senior U.S. official said.

The top U.S. diplomat "commended the work of the coalition taking military action against the Huthis" and noted Washington's support "including intelligence sharing, targeting assistance, and advisory and logistical support for strikes against Huthi targets," the State Department official said.

More than 1,000 civilians have been killed in Boko Haram attacks in the first three months of this year, Human Rights Watch said on Thursday.

"Each week that passes we learn of more brutal Boko Haram abuses against civilians," the group's Nigeria researcher, Mausi Segun, said in an emailed statement.

Saudi-led strikes on Yemeni rebels are set to dominate an Arab summit that will consider forming a joint military force as several countries in the region teeter on the brink of collapse.

For years Arab states had contented themselves with directing proxies in regional struggles.

Japanese Foreign Minister Fumio Kishida could visit Cuba next month, a report said Thursday, becoming one of the highest-profile visitors to the isolated island since a thaw began between Washington and Havana.

If the visit goes ahead, Kishida is expected to call for closer ties with Cuba, where Japanese companies can invest in infrastructure projects and mineral resource development, Japan's public broadcaster NHK said.

The United States fears that Moscow and pro-Russian separatists will reignite the armed conflict in eastern Ukraine following a February ceasefire that has largely held, a U.S. diplomat warned Wednesday.

"Russia can reignite the conflict at any time of its choosing," the senior State Department official told journalists.

A nuclear deal with Iran capping over a decade of talks is in sight by a March 31 deadline, U.S. officials said, but they remained cautious as top diplomat John Kerry prepared Thursday for down-to-the-wire negotiations.

Kerry will push Iran to agree by Tuesday on the outlines of the long-elusive deal disabling parts of its nuclear infrastructure, in return for an easing of crippling global sanctions, at renewed talks in the Swiss lakeside town of Lausanne.