DAMASCUS - More than 148 people were killed Monday in bombings claimed by the Islamic State group in northwestern Syria, the deadliest attacks to date in the regime's coastal heartland.

Seven near-simultaneous explosions targeted bus stations, hospitals and other civilian sites in the seaside cities of Jableh and Tartus, which until now had been relatively insulated from Syria's five-year war.

The attacks on strongholds of President Bashar al-Assad's regime came as IS faces mounting pressure in Syria and neighbouring Iraq, where a major offensive to retake the jihadist-held city of Fallujah is underway.

A hundred people were killed in Jableh and another 48 in Tartus to the south, including children, said the Syrian Observatory for Human Rights monitor. Observatory head Rami Abdel Rahman said they were "without a doubt the deadliest attacks" on the two cities since the start of the war.

IS claimed the blasts in a statement, saying they were in retaliation for Syrian regime and Russian air strikes against the jihadists and vowed to carry out "more devastating and bitter attacks".

IS is not known to have a presence in Syria's coastal provinces, where its jihadist rival, the Al-Qaeda-affiliated Al-Nusra Front, is much more prominent. But IS, notorious for using deadly sleeper cells to attack its enemies, suggested the creation of a "wilayat al-Sahel" or Islamic province for the coastal area. "I'm shocked, this is the first time I hear sounds like this," said Mohsen Zayyoud, a 22-year-old student in Jableh.

'First time we see dead people'

In Tartus a 42-year-old bank employee was just as stunned. "It's the first time we hear explosions in Tartus, and the first time we see dead people or body parts here," said Shady Osman.

Jableh lies in Latakia province, while Tartus is the capital of the adjacent governorate of the same name. Both have remained relatively secure even as the war has raged in Latakia province's rural northeast and throughout the country.

The early morning attacks began at a bus station in Tartus, where regime ally Russia has long maintained a naval facility. The Observatory said a car bomb exploded and as people began to flock to the site two suicide bombers detonated explosive belts.

A police source in the city confirmed a car bomb had hit the entrance to the station and two suicide bombers attacked inside.  Footage from state television showed charred mini-buses and others ablaze. Around 15 minutes after the Tartus blasts, four explosions rocked Jableh further north.

Blast inside hospital

The Observatory said a car bomb and three suicide attackers targeted a bus station, a hospital, and a power station. One attacker detonated explosives inside the emergency room of the state-run hospital after carrying victims of the first attack there.

A police officer said a car bomb also targeted the Asaad hospital in the city. The attacks drew condemnation from around the world, while Human Rights Watch warned that targeting civilians "would appear to constitute war crimes."

UN chief Ban Ki-moon denounced the "terrorist attacks" while France called the bombing heinous. The United States expressed strong condemnation as well and vowed to continue its military campaign against IS jihadists in Syria and Iraq.

US Secretary of State John Kerry also spoke to Russian counterpart Sergei Lavrov by phone and urged him to press the Syrian regime to halt its attacks on Aleppo in the north and Daraya near Damascus.

State Department spokesman Mark Toner said Russia "has a special responsibility" to rein in Assad's forces. Russia said Lavrov and Kerry discussed its proposal to launch joint air strikes targeting "terrorist groups".

"Russia has a special responsibility in this regard to press the regime to end its offensive attacks and strikes that kill civilians, immediately allow relief supplies, as determined solely by the UN, to reach all in need, and to comply completely with the cessation of hostilities," Toner said.

The United States and Russia are co-partners in the so-called Vienna diplomatic process of the International Support Group for Syria, which met last week in the Austrian capital but made no notable progress.

The 20 world and regional powers taking part in the process have so far failed to turn a fragile cessation of hostilities in Syria, in effect since February 27, into a durable truce between the government and opposition groups.

Toner said the regime of President Bashar al-Assad was using air strikes and attacks on civilians to gain tactical advantage.

The Kremlin said the attacks "demonstrate yet again how fragile the situation is in Syria and the need to take energetic measures to relaunch peace talks."

He said the United States is looking to Russia to provide the pressure needed to get the regime "to reconsider the fact that if this keeps up, we may be looking at a complete breakdown of the cessation."

World powers have struggled to rekindle UN-brokered peace negotiations which fizzled in April when Syria's opposition walked away in frustration at stalling progress on the country's dire humanitarian situation.

Syria's conflict has evolved from a popular uprising to a multi-faceted war that has killed more than 270,000 people and displaced millions.

IS seized control of large parts of Syria and Iraq in mid-2014, declaring an Islamic "caliphate" and spreading its influence. It has claimed deadly attacks in the West and throughout the Middle East, including twin bombings on military forces in Yemen's second city of Aden on Monday that killed at least 41 people.

Monday's bombings were reminiscent of a string of attacks in 1986 in northwestern Syria -- including in Tartus -- that killed 144 people and which Syrian officials blamed on the regime of Saddam Hussein in neighbouring Iraq.