Syrian troops on Thursday entered the ancient city of Palmyra and Iraqi forces launched an offensive against Mosul as pressure mounted on key strongholds of the Islamic State jihadist group.

US Secretary of State John Kerry was meanwhile holding talks in Moscow to push for peace efforts in Syria, as UN envoy Staffan de Mistura said in Geneva that he was aiming to restart talks on April 9.

Backed by Russian warplanes and allied militia on the ground, the Syrian army advanced into Palmyra after launching a desert offensive early this month, said the Syrian Observatory for Human Rights.

The defence ministry in Moscow said Russian aircraft carried out 146 strikes on "terrorist targets" in the Palmyra area between Wednesday and last Sunday.

A total of 320 "terrorists" were killed, it said, adding that six command posts, five tanks, two ammunition dumps and 15 vehicles were among the targets "eliminated".

News agencies in Moscow said later that a Russian special forces officer was killed while directing air strikes near Palmyra.

IS overran Palmyra -- known as the "Pearl of the Desert" -- last May and it has since blown up UNESCO-listed temples and looted relics that dated back thousands of years.

"Regime forces have entered the Hayy al-Gharf neighbourhood in the southwest of Palmyra. They are advancing very slowly because of mines planted by IS," Observatory director Rami Abdel Rahman said, adding troops were also advancing from the north.

A Syrian military source said the army had entered Palmyra from the northwest after seizing control of part of the historic Valley of the Tombs.

"The clashes, which are ongoing, are fierce," he told AFP.

- 'Nightmare is nearly over' -

Syria's antiquities chief Maamoun Abdelkarim hailed the "imminent" recapture of Palmyra, vowing to rebuild monuments the jihadists destroyed.

"I am so happy that the liberation is imminent... and that the nightmare is nearly over, before it is too late, before the total destruction of the ancient city," he said.

"I think this 10-month period has been the worst of our lives."

IS had earlier urged civilians to flee the city, where about 15,000 of its 70,000 original residents remained after the jihadist takeover.

Palmyra's recapture would be a major strategic and symbolic victory for President Bashar al-Assad, since whoever holds it also controls the vast desert extending from central Syria to the Iraqi border.

UNESCO chief Irinia Bokova welcomed the push to recapture Palmyra.

"For one year, Palmyra has been a symbol of the cultural cleansing plaguing the Middle East," Bokova said, adding that she welcomed the effort to retake the "martyr city inscribed on the UNESCO world heritage list".

Across the border, Iraqi forces announced the start of an offensive to retake the second city of Mosul, IS's main hub in Iraq.

The army and the Popular Mobilisation paramilitary force "have begun the first phase of conquest operations" in the northern Nineveh province, of which Mosul is the capital, Iraq's joint operations command said.

It said four villages had been taken between the town of Qayyarah, which is held by IS, and Makhmur, where US-backed Iraqi forces have been massing.

Prime Minister Haider al-Abadi congratulated the fighters for "launching the operation to retake Nineveh and achieving their first success with the recapture of some villages."

- Rare talks with Putin -

The army did not say how long this phase of the operation was expected to take and Iraqi forces still look far from being in a position to take the city itself.

Experts have warned any battle to retake Mosul would be difficult, given the significant number of jihadists and civilians in the city and the time IS has had to prepare defences.

In Moscow, Kerry was to see if President Vladimir Putin could be convinced to support an end to Assad's rule in Syria.

"We... have some ideas about how we can together, most effectively, make progress even deeper and begin the very serious political process of transition," Kerry told Putin.

Tuesday's bombings in Brussels "underscore the urgency" of fighting IS and other extremist organisations, he added.

The Moscow meetings came as UN envoy de Mistura met the government delegation to Geneva, on the final day in the current round of indirect peace negotiations.

Syria's main opposition said the talks "laid the basis" for substantive negotiations next month, but warned any real progress depended on Russia pressuring the government.

"This is a unique moment, a precious moment and we hope Russia will seize this moment and use its leverage," High Negotiations Committee spokeswoman Bassma Kodmani said.

Assad's future has been a key obstacle in the latest talks, with the government insisting any discussion of him leaving is "excluded" and the opposition saying any talk of allowing him to stay is "absolutely unacceptable".

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