Damascus (AFP) - The Islamic State group suffered a double setback in Syria Friday as army troops recaptured half of the ancient city of Palmyra and the Pentagon said the jihadists' second-in-command was killed in a US raid.

The seizure by Russian-backed Syrian troops of half of Palmyra including the hilltop citadel and the airport came nearly a year after IS overran the UNESCO world heritage site.

The Syrian regime's gains came after US Secretary of State John Kerry and Russia's President Vladimir Putin agreed to intensify the drive for a political settlement in the war-torn nation.

Pentagon chief Ashton Carter said the death this week of Abd ar-Rahman Mustafa al-Qaduli, described as IS's number two, would hamper the jihadists' ability to conduct operations in Iraq, Syria and elsewhere.

He said the US was "systematically eliminating" IS's cabinet, referring also to the killing earlier this month of "Omar the Chechen", described by Washington as the jihadists' defence minister.

"The momentum of this campaign is now clearly on our side," said Carter.

However, separate attacks by IS in Iraq and Yemen Friday claimed over 50 lives, with observers warning that as their self-proclaimed "caliphate" shrinks towards extinction, its fighters are likely to ramp up suicide attacks on civilian targets.

A suicide bomber killed at least 30 people at a local football tournament south of Baghdad, while in Aden three suicide bombings at security checkpoints killed 22 people, including 10 civilians.

- 'An ancient treasure' -

Syrian state television said loyalist troops seized the Palmyra citadel "after inflicting many losses in the ranks of the terrorist group Daesh," using another name for IS.

It said the army had also cut off the main Palmyra-Deir Ezzor highway leading to the Iraqi border.

"Pro-government forces, which have the support in Palmyra of the Russian air force, took control of half of the city as well as the airport," a military source said.

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IS has blown up UNESCO-listed temples and looted relics that dated back thousands of years. Built in the 13th century, the citadel is Palmyra's main Islamic-era monument.

Syria's antiquities chief Maamoun Abdelkarim said the army had also "liberated the district of hotels and restaurants as well as the Valley of the Tombs".

He said troops were 600 metres (yards) from the site of the Temple of Bel, which IS destroyed in September, "but it is advancing slowly because of mines and above all to protect the city, which is an ancient treasure."

- Assad still a sticking point -

The Britain-based Syrian Observatory for Human Rights said 18 soldiers were killed in fighting and by mines planted by retreating jihadists, while 24 IS militants died in air strikes and clashes.

In Moscow, the defence ministry said Russian warplanes carried out 146 air strikes against "terrorist" targets in Palmyra from March 22 to 24.

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

Despite the football tournament bombing, IS faces mounting pressure in Iraq where the army said on Thursday it had launched a long-awaited offensive to retake second city of Mosul, a key IS hub since 2014.

Iraqi forces cleared roadside bombs and booby traps Friday in villages from which they ousted jihadists a day earlier south of Mosul, officials said.

Friday's fighting for Palmyra came as the latest round of peace talks aimed at ending Syria's five-year war, which has left more than 270,000 people dead, came to a close.

Kerry and Putin, who back different sides in Syria's war, agreed at a rare meeting in Moscow to push for a political settlement, but the future of Assad remains a sticking point.

In Beirut, UN chief Ban Ki-moon urged a speedy end to the war and other Middle East conflicts saying "this is a critical time for the region".

But unless Assad agrees to step down, there are concerns the Syrian opposition could drop out of UN-brokered peace negotiations, which UN envoy Staffan de Mistura aims to restart on April 9.

- 'Jihadists celebrating Brussels' -

The focus of Syria's war appears to have shifted to the battle against IS in Palmyra, nearly a month after a truce between the army and non-jihadist rebels brokered by the United States and Russia came into force.

Global concern over the jihadist threat was further heightened this week by a deadly attack in Brussels that was claimed by IS.

In IS's de facto capital Raqa in northern Syria, "jihadists have been celebrating the Brussels attack all week," said Observatory director Rami Abdel Rahman.

The group's top religious figure in the city, Abu Ali al-Sharii, led the Friday prayer with a pledge to commit more violence.

"We vow new operations by jihadists in the West," he said, according to the Observatory.