By Parisa Hafezi and Maya Gebeily

DUBAI/BEIRUT, March 12 (Reuters) - Iran will fight on and keep the Strait of Hormuz shut as leverage against the United States and Israel, new Supreme Leader Mojtaba Khamenei said on Thursday in defiant first comments attributed to him since he succeeded his slain father.

Hours later, Israeli Prime Minister Benjamin Netanyahu used his first press conference since the start of the war to make his own strong statement, issuing a veiled threat to kill Khamenei and defending the military assault on Iran.

By Laila Bassam, Ahmed Rasheed, Pesha Magid and Tom Perry

BEIRUT/BAGHDAD/JERUSALEM, March 12 (Reuters) - Shi'ite Muslim armed groups in Lebanon and Iraq are stepping up their role in the war with the U.S. and Israel, showing the Iran-backed "Axis of Resistance" can still wage attacks despite damage inflicted on the alliance during the Gaza conflict.

Groups that have long been armed and financed by Iran and loyal to its Shi'ite Islamist rulers are now helping Tehran intensify the war around the region, strikes in recent days show.

By Ahmed Kerdi, Maya Gebeily and Maayan Lubell

BEIRUT/JERUSALEM, March 12 (Reuters) - Israeli airstrikes hit two buildings in the heart of Beirut near the Lebanese government's headquarters on Thursday, ramping up Israel's offensive against Iran-backed Hezbollah and dragging Lebanon deeper into the Middle East war.

A day after Iran-backed Hezbollah launched its biggest rocket salvo of the conflict, Israel's defence minister said the military had orders to expand the campaign and Israel's military chief said the operation in Lebanon would not be short.

LONDON, March 12 (Reuters) - The U.S. Navy, perhaps with an international coalition, will escort vessels through the Strait of Hormuz when it is militarily possible, U.S. Treasury Secretary Scott Bessent told Sky News in an interview on Thursday.

"My belief, that as soon as it is militarily possible, the US Navy, perhaps with an international coalition, will be escorting vessels through," Bessent said.

March 12 (Reuters) - The U.S. and Western allies clashed with Russia and China on Thursday over Iran's nuclear intentions, as Washington sought at the United Nations to further justify the war it launched on Iran two weeks ago.

At a meeting of the 15-member U.N. Security Council, which is chaired this month by the U.S., Russia and China moved unsuccessfully to block a discussion about a committee established to oversee and enforce U.N. sanctions on Iran. They were overruled 11-2 with two abstentions.

The world has entered a "new dark age of abuses", with the United States "raining death" on Iran and Venezuela, a UN special rapporteur said Thursday.

Ben Saul, the United Nations' special rapporteur on the promotion and protection of human rights and fundamental freedoms while countering terrorism, said the war in the Middle East would not improve global security.

"We have entered a new dark age of abuses in the name of countering terrorism," he told a press conference in Geneva.

OTTAWA, March 12 (Reuters) - The foreign ministers from the Group of Seven will have detailed discussions on diplomatic efforts related to the war in Iran when they meet near Paris at a scheduled meeting at the end of March, Canada's foreign minister said on Thursday.

France, which is currently chairing the G7, will host the first of two foreign ministers meeting on March 26-27 at the Vaux de Cernay abbey, some 50km southwest of Paris.

"We will be having a full-fledged conversation about the diplomatic efforts relating to the war," Anita Anand told reporters on Thursday.

AFP reporters have been speaking to a range of people living in Iran to build a picture of life under daily US and Israeli bombardment despite an internet blackout and major communication problems.

Here is a selection of their comments, either from people who sent messages to AFP reporters or from those interviewed at the borders as they fled.

AFP is withholding identifying information for their security.

- My son 'has no understanding of war' -

A 37-year-old man in Bukan, northwest Iran:

By Simon Lewis

WASHINGTON, March 12 (Reuters) - The Trump administration has organized nearly 50 flights to return U.S. citizens from the Middle East since the U.S.-Israeli war with Iran began, a State Department official said on Thursday, and officials said demand for the flights has declined.

The State Department was criticized in the days after the conflict began on February 28 for its late preparations to relocate diplomats and offer assistance to U.S. citizens as Iran's strikes led to airspace closures and flight cancellations across the region.

By Daphne Psaledakis

WASHINGTON, March 12 (Reuters) - An FBI alert to law enforcement agencies last month warning of the possibility that Tehran might try to retaliate for any U.S. strikes on Iran by launching drone attacks in California was based on a single unverified tip, White House press secretary Karoline Leavitt said on Thursday, adding that there has never been such a threat to the U.S. from Iran.