WASHINGTON - US President Barack Obama voiced renewed determination to destroy the Islamic State group Monday, vowing to win back territory in the Middle East and kill the group's leaders.

Sounding a notably more strident tone, Obama said that the United States and its allies were taking the fight to Islamic State in Iraq and Syria.

"We are hitting them harder than ever," said Obama, in a second address following a seemingly IS-influenced attack in San Bernardino that has raised questions about his strategy.

"As we squeeze its heart, we will make it harder for ISIS to pump the propaganda to the rest of the world," Obama insisted at the Pentagon after meeting with US military leaders.

Listing a series of Islamic State leaders killed in coalition operations, Obama issued a stern warning. "ISIL leaders cannot hide," he said. "Our next message to them is simple, you are next."

Even before the attack by a Muslim husband and wife in California killed 14 people, polls showed that more than 60 percent of Americans disapproved of the way Obama is handling the Islamic State and the broader terror threat.

As was the case in his primetime Oval Office address a week ago, Obama offered no shift in policy, but admitted: "We are recognizing that progress needs to keep coming faster."

Barack Obama has vowed to push ahead with plans to close the Guantanamo Bay prison for terror suspects, despite evidence that a substantial number of former detainees have returned to the battlefield.

In an interview with Yahoo News that was published Monday, Obama said the strategic benefits of closing the notorious facility outweighed incidents of recidivism among "low-level" former inmates.

The White House itself admits that around 10 percent of those released from Guantanamo have resumed fighting for Islamic extremist organizations, but says it is more important to shutter a facility that has become a recruiting tool for militants.

Meanwhile, US Secretary of State John Kerry will use talks in Moscow on Tuesday to try to narrow differences with Russian leader Vladimir Putin over the role of Syrian President Bashar al-Assad in any political transition, a senior State Department official said on Monday.

He will also seek to prepare the ground for a third round of talks of world powers on Syria amid doubts over whether a meeting pencilled in for Friday in New York will go ahead.

Russia's foreign ministry said late on Monday that Kerry and Russian Foreign Minister Sergei Lavrov had agreed in a phone call on the need for specific preconditions to be met before any new meeting, throwing its timing into doubt.

Russia is one of Assad's staunchest allies and launched a campaign of air strikes to support his forces against insurgents on Sept. 30. It says only the Syrian people and not external powers should decide his political fate.

Speaking in Paris before Kerry visits Moscow on Tuesday, the State Department official said Kerry would raise concerns about Russia's continued bombing of Syrian opposition forces instead of Islamic State militants, an approach likely to anger Moscow.

Ahead of the talks, the Russian foreign ministry issued a statement criticising US policy on Syria, complaining that Washington was not ready to fully cooperate in the struggle against Islamic State militants and needed to rethink its policy of "dividing terrorists into good and bad ones".

Kerry's meeting with Putin follows a meeting last week in Riyadh which agreed to unite a number of opposition groups excluding Islamic State to negotiate with Damascus in Syrian peace talks.

While Kerry said there were still "kinks" that needed to be worked out on the plan to unite the opposition groups, the Kremlin rejected the outcome of the Riyadh meeting, saying it had no right to speak on behalf of the entire Syrian opposition.

Assad himself appeared to cast doubt on the very idea of peace talks last Friday, saying he would not negotiate with armed groups which he said were backed by the United States and Saudi Arabia.

The opposition groups said Assad should leave power at the start of a transitional period.

"We don't have a full meeting of the minds yet (on Assad), we will talk about some of the details of a transition...in the hopes of narrowing the differences between us," a senior State Department official told reporters.

While in Paris on Monday, Kerry met with counterparts from Saudi Arabia, Qatar and Jordan as he prepared for the Moscow talks.

His talks with Putin, and a lengthier meeting in the morning with Lavrov, will delve deeper into details of a planned Jan. 1 ceasefire in Syria, as well as recent statements by Russia on supporting the Free Syrian Army, which has been fighting Russian- and Iranian-backed government forces.

"We'll be interested to hear what the Russians have in mind there, given the Free Syrian Army's concern about how Assad has been treating his own people," the official added.

Russia has given mixed messages on the FSA with news agencies on Monday quoting Valery Gerasimov, the chief of the Russian army's general staff, as saying his country was providing it with weapons, ammunition and material support, and a top Kremlin aide later saying there was no such arrangement.

Free Syrian Army (FSA) rebels fighting Assad's forces in western Syria denied receiving any support from the Russian air force, saying on the contrary it continued to bomb them.

Kerry, during his meetings, will also underscore the importance of implementing the Minsk peace accords on Ukraine and plans for free and fair elections in eastern Ukraine.

Despite cooperation with Russia on a political transition in Syria, the United States maintains economic sanctions against Moscow for its annexation of Ukraine's Crimea region and support for separatists in eastern Ukraine.

"We have made absolutely clear all the way through this at every level that we are not playing 'lets make a deal here' trading Ukraine for Syria, that these are distinct issues with distinct paths forward," the official added.