Beirut (AFP) - The Islamic State jihadist group has made several high-profile advances in recent days, despite a US-led air campaign against the movement in Syria and Iraq.

Here are answers to some key questions about the group's progress and the international effort to stop them:

Q: Where has IS advanced recently?

A: On Sunday, Islamic State group fighters seized Ramadi, the capital of Iraq's largest province of Anbar, in their biggest victory since a major offensive in Iraq last summer.

The group has also moved to within a kilometre (less than a mile) of Syria's Palmyra world heritage site, and has seized the town of Al-Sukhnah and two gas fields northeast of the ancient city.

The advances expand the existing territory the group holds across Syria and Iraq -- land it has labelled an Islamic "caliphate".

Q: What is the significance of these advances?

A: The fall of Ramadi is a particular blow to Iraq's government, which only last month was touting its recapture of the city of Tikrit from the jihadist group.

In Syria, Palmyra has key symbolism as both the home of world-famous Greco-Roman ruins, but also the site of one of the regime's most infamous prisons.

Homs province, in which Palmyra lies, also holds several large gas fields, including two -- Arak and Al-Hail -- that were captured by IS on Monday.

IS has proven adept at filling its coffers by exploiting captured oil and gas resources in Syria and Iraq.

Jessica Lewis of the Institute for the Study of War think-tank said the two offensives could be linked, and be a bid by IS to consolidate its territory in eastern Syria and western Iraq.

Q: What is the US-led coalition doing?

A: A coalition led by the United States has been carrying out air strikes against IS since last year in both Iraq and Syria.

The strikes have targeted IS's military equipment, oil fields and refineries and fighters on the ground.

The raids have had some notable successes, including preventing the group from advancing on the capital of the Kurdish region in northern Iraq, Arbil.

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They have also managed to push back IS from the Syrian Kurdish town of Kobane on the Turkish border, and were key to the recapture of Tikrit by Iraqi government forces.

Q: Why is IS still advancing?

A: Analysts note that the coalition's victories against IS have come in areas where its strikes have been backed by allied ground forces.

"It's a basic truism of counterinsurgency that success requires boots on the ground," said Max Abrahms, a political science professor at Northeastern University in Boston, Massachusetts.

In Kobane the coalition worked with Kurdish fighters, and in Tikrit it was backed by Iraq's army and allied paramilitary groups.

The dynamics are different in Ramadi, where the Shiite militias were not deployed, and in Palmyra, where Syrian President Bashar al-Assad's forces are on the ground.

Washington has said it will not coordinate its strikes in Syria with Assad's forces and has ruled out deploying ground troops to take on IS.

Ramadi is also part of Iraq's "Sunni heartland... where the Sunni community has not completely rejected IS," said analyst Ayham Kamel, Middle East and North Africa director at the Eurasia Group.

"It is not necessarily approval of IS, it could be fear or hedging, but they are not rising against IS."

Q: What comes next for IS?

A: The group's advances put paid to US claims that IS is "on the defensive," after losses including Tikrit and a failed attempt to seize and hold the Yarmuk Palestinian camp in southern Damascus.

And in cases where the US-led coalition cannot rely on ground forces, IS is likely to score additional successes.

Abrahms also said the coalition had been largely reactive, bringing its firepower to bear only after an area had already fallen to IS.

On Monday, the Pentagon acknowledged Ramadi's capture as a "setback" but said the war on IS would involve "ebbs and flows".

For IS, however, even small advances will allow them to claim continuing power, said Aymenn Jawad al-Tamimi of the Middle East Forum research group.

"The group's slogan is 'baqiya wa tatamaddad' (remaining and expanding), and they might not always be expanding, but they are certainly remaining."