Tensions remained high in Syria on Thursday as Russia continued to back the Syrian government in its latest major offensive against Islamic State (IS) militants while NATO defense ministers meeting in Brussels used harsh language against Moscow for escalating the civil war.

Syria's army chief of staff, General Abdullah Ayoub, said Thursday that the military had unleashed a broad offensive against rebel-held areas across the country, buoyed by Russian air support, according to the state news agency SANA.

"Today the Syrian armed forces have started a wide-scale offensive with the aim of eliminating the terrorists' positions and liberate the areas and towns that have suffered from terrorism and its woes and crimes," Ayoub said in a statement.

The senior Syrian military officer said Russia's air strikes have weakened the rebel groups, while the army now can have a stronger position.

In a show of force, the Russian defense ministry said missiles fired from its ships in the Caspian Sea hit weapons factories, arms dumps, command centers and training camps supporting IS forces.

Russia's air force hit 27 IS targets overnight in the Syrian provinces of Homs, Hama and Raqqa, the Interfax news agency quoted the Russian defense ministry as saying on Thursday.

The military gain by Syrian troops with Russian assistance contrasted with the ongoing air strikes by the US-led coalition that started about a year ago, but that has seen little progress. Iraq is also considering asking Russia to help it fight IS groups on its territory, according to media reports Wednesday.

In an apparent competitive gesture, the US-led coalition said in a statement released on Thursday that the coalition on Wednesday also launched 18 strikes against the IS in Iraq and two in Syria.

NATO threatens 

Beside that, Western countries are still smarting from Russia's weekend incursions into Turkey's airspace near northern Syria.

"NATO is ready and able to defend all allies, including Turkey against any threats," NATO's Secretary-General Jens Stoltenberg told reporters as he arrived for the meeting Thursday.

"NATO has already responded by increasing our capacity, our ability, our preparedness to deploy forces including to the south, including in Turkey," he said, noting that Russia's air and cruise missile strikes were "reasons for concern."

As Russian and US planes fly combat missions over the same country for the first time since WWII, NATO is eager to avoid any international escalation of the Syrian conflict.

The incursions of two Russian fighters in Turkish airspace on Saturday and Sunday has brought the Syria conflict right up to NATO's borders, testing the alliance's ability to deter a newly assertive Russia without seeking direct confrontation.

US-led coalition aircraft bombing Islamist militants in Syria were re-routed at least once in the last six days to avoid a close encounter with Russian planes, a Pentagon spokesperson said on Wednesday.

"We've had an instance at least where there's been action taken to make sure we didn't have an unsafe separation of space," said Captain Jeff Davis, a Pentagon spokesperson. "We have had to do re-routing of the course of an airplane."

"Russia is making a very serious situation in Syria much more dangerous," Britain's defense minister, Michael Fallon said, calling on Moscow to use its influence on Syrian President Bashar al-Assad to stop bombing civilians.

For 40 years, NATO's central task was deterring Russia in the east during the Cold War, but now, after a decade-long involvement in Afghanistan, the alliance is facing a reality-check close to home, with multiple threats near its borders.

Divisions between eastern NATO members, who want to keep the focus on the Ukraine crisis, and others who fret about IS militants, risk hampering a unified response from the 28-nation North Atlantic alliance.

China calls for Geneva III

Chinese foreign ministry spokesperson Hua Chunying said on Thursday that China has always backed a political solution to the Syrian crisis, and called for a new Geneva conference on Syria.

"We have noticed Russia said the military campaign is at the invitation of the Syrian government and aims to combat terrorism there," Hua said at a press briefing when asked if China backs Russia's air strikes against IS terrorists in Syria.

China has always believed a political solution is the best way to end the unrest in Syria, and it calls for an equal, inclusive and open political dialogue on the matter soon, she said.

Specifically, China is calling for a third UN-backed Geneva Conference on Syria, Hua said. The previous two were held in 2012 and 2014.