At least 43 people died in the blasts, and 45 were also wounded in the double bombing outside of Tartus city. It was followed by another suicide bomber who was wearing an explosive belt.
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The wave of attacks came as the extremist group has steadily lost ground in both Syria and Iraq in recent months.
The Islamic State group launched a series of suicide bombings and other attacks across Syria on Monday, killing at least 48 people as it targeted government-held areas and Kurdish forces.
The blasts happened in government-held areas of Damascus, Homs and Tartous, as well as at a Kurdish checkpoint in Hassakah.
Islamic State fighters carried the out suicide attacks, its Amaq news agency said on Monday.
Conflicting casualty figures are common in the 5-year-old Syria civil war.
State news agency SANA reports that twin blasts at the entrance to coastal city of Tartus Monday morning killed at least 30 people.
The Observatory and a city hospital put the death toll at 35, including members of the Syrian military, and said the number was likely to rise.
A auto bomb struck a bridge on the global coastal motorway leading to Tartus and suicide bomber then struck the crowd that had gathered there, according to Sana, which said 35 people were killed. The blasts hit during a summer festival at Tartous, whose beaches recently featured in a government tourism video.
Most were killed in the attacks in Tartous, house to the primary Russian airbase in Syria. The Observatory said it hit an army checkpoint, killing four officers.
A source at Damascus Countryside Police Command said that the terrorist bombing that took place on the road of al-Sabboura/-al-Bajja’ claimed the life of a person and injured three others.
A local police chief told state media three men were stopped in a vehicle by security forces.
The suicide auto bomber had apparently planned to detonate inside the city but did so at the entrance after he was discovered by the authorities, the report said.
The head of the observatory, Rami Abdel Rahman, said that the blast in al-Hassakeh hit a checkpoint belonging to the Kurdish Asayesh security forces.
The militia group, a vital component of a US-supported campaign against the Islamic State, took over Hasaka in August.
The YPG already controls swathes of northern Syria where Kurdish groups have established de facto autonomy since the start of the Syrian war in 2011, much to the alarm of neighbouring Turkey.
Observatory director Rami Abdel Rahman said the blasts appeared to be coordinated.
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The reports indicated that the attackers were killed and nearly simultaneously another explosion occurred when several people tried to help the wounded in the shooting.
