Syrian activists say a series of explosions have killed at least 148 people in two strongholds of President Bashar al-Assad's regime on the Mediterranean coast.
The Islamic State (IS) group claimed responsibility for the May 23 attacks in the city of Tartus and Jableh, a town to the north.
The British-based Syrian Observatory for Human Rights said at least 100 people were killed in Jableh and another 48 in Tartus.
Observatory head Rami Abdel Rahman said they were "without a doubt the deadliest attacks" on the two cities since the start of the war.
State news agency SANA said two bus stations, a hospital, and other facilities were targeted.
Russia, a key ally of Assad, has a small naval base in Tartus and an air base near Jableh, from where it has conducted air strikes across Syria.
The two cities have been relatively insulated from the war raging in Syria, which has killed at least 270,000 people since March 2011.
The Kremlin expressed concerned over the bomb attacks, saying rising tension in Syria underscores the need to continue peace talks.
A truce was brokered by the United States and Russia in February, but the agreement has since unraveled.