A wave of coordinated bombings in Syria claimed by Isis (Islamic State) killed at least 78 people in the usually calm coastal area where Russian troops are based, Syrian state media reported.
The attacks targeted the cities of Tartus and Jabla in a part of the country that has remained loyal to Syrian President Bashar al-Assad throughout Syria's five-year conflict and where Russia maintains military bases in support of government forces.
It marked the worst single day of violence yet in the Mediterranean coastal region and the first time Isis has carried out suicide attacks there. That raised fears of possible further bombings in government-held areas that have remained relatively insulated from the war raging elsewhere.
One group monitoring the conflict estimated the death toll at dozens higher than the count given by Syrian officials.
Russia said the bombings demonstrate Syria's continued fragility. A nearly three-month-old ceasefire effort has gradually disintegrated, and negotiations aimed at securing a peace settlement remain stalled.
"Naturally, growing tensions and such terrorist activities cannot but cause increased worries," Russian presidential spokesman Dmitry Peskov told reporters in Moscow, adding that the coordinated attacks underlined "the necessity of continuing active steps toward the peace process".
Amaq, a news site linked to Isis, claimed the militant group was responsible for the blasts. They coincided with an escalated campaign of bombings across the region by the group as it is steadily pushed back from strongholds in Syria and Iraq.
Syrian state television, however, blamed an Islamist rebel group, Ahrar al-Sham, and claimed that the group is backed by the United States.
The accusation may represent an attempt to push the US Government to accede to Russian demands that it designate Ahrar al-Sham as a terrorist organisation.
Obama Administration officials say they have no direct contacts with Ahrar al-Sham, which is tied to the wider rebel movement and also al-Qaeda through a complex network of alliances. But the United States has consistently rejected Russian requests to label it as terrorist. An Ahrar al-Sham official visited Washington in December, according to a report by the McClatchy news organisation at the weekend.
The official, Labib Al-Nahhas, denied that Ahrar al-Sham carried out today's attacks. "This is not our modus operandi: we do not target civilians," he said in a statement posted on Twitter.
The attacks appeared to be a well-planned and coordinated operation in which suicide bombers wearing vests and at least one car bomb detonated almost simultaneously in two cities.
According to state media, two or three explosions struck the port city of Tartus, where Russia has long maintained a naval base, and four more hit Jabla. Russia established an air base in the countryside outside Jabla when it intervened in Syria's war last year, but there was no indication either of the Russian bases was the target.
The sites that were attacked could have been expected to be crowded with civilians at the time, and civilians appeared to have been the main target.
The blasts in Jabla struck a busy bus station, a hospital's emergency entrance and a government electricity office, while those in Tartus also hit a bus station, as well as a residential neighborhood nearby.
State television broadcast footage of dozens of mangled buses and cars and pools of blood at a wrecked cafeteria at the Jabla bus station.
The Britain-based Syrian Observatory for Human Rights, which tracks the Syrian conflict through a network of activists and other sources, said the toll could be as high as 121 dead, with 73 in Jabla and 48 in Tartus.
The coastal area is populated mostly by members of Assad's minority Alawite community who have remained loyal to him throughout the war, sparing the region from the battles that have destroyed many other parts of the country.
The blasts come two days after the top Islamic State spokesman, Abu Mohammed al-Adnani, urged followers to ramp up terrorist attacks against the group's enemies as the holy month of Ramadan approaches and in response to battlefield setbacks.
Some of the blows to the group in Syria have been inflicted by Syrian government troops, Shiite militias and Iranian advisers, backed by Russian airstrikes. The militants were ejected from the historic city of Palmyra in March and are locked in battle with Syrian troops in the eastern town of Deir al-Zour.
In Iraq, government forces - in collaboration with some of those same Shiite militias - have launched attacks to recapture Fallujah, a key city west of Baghdad. A change of hands in Fallujah, which the Islamic State has controlled for 2½ years, would be a major gain for Iraqi forces.