BEIRUT -- More than 2,000 Syrians -- almost half of them pro-government forces -- have been killed in just over two weeks of fighting in Syria, making it one of the worst death tolls in the country's civil war, opposition activists said Monday.

The report reflects a recent surge in deadly attacks by the al-Qaida-breakaway Islamic State group targeting government forces, signalling shifting priorities as Sunni militants seek to consolidate power in areas under their control.

President Bashar Assad's forces have gained momentum in the fighting with rebels seeking to topple him from power. Infighting also has hurt the rebel cause, with Islamic extremists battling moderate fighters.

The recent attacks came after Assad was re-elected last month to a third, seven-year term in a vote that was confined to government-controlled areas and dismissed by the opposition and its Western-allies as a sham. In his inauguration speech on July 16, he confidently declared victory and praised his supporters for "defeating the dirty war" against Syria.

Assad prayed at a Damascus mosque at the start of a major Muslim holiday on Monday, smiling as he was shown on state TV greeting Muslim clergymen at the Khair mosque in his second public appearance in less than two weeks.

Since his inauguration, fighters from the Islamic State group have launched attacks against army positions in three different provinces in northern and central Syria. In the past week alone, the militants captured a government-controlled gas field and two major army bases in three different provinces.

More than 300 soldiers, guards and workers at the Shaer field were reported killed by Islamic State militants in a three-day blitz offensive to capture the field. The army recaptured Shaer at the weekend.

Militants last week also overran the sprawling Division 17 military base in the northern Raqqa province, killing at least 85 soldiers inside. Amateur videos posted online by activists showed more than a dozen beheaded bodies in a busy square said to be in Raqqa. Some of the heads were placed on a nearby fence, where at least two headless bodies were crucified. The videos appeared genuine and corresponded to other AP reporting of the events.

On Sunday, the militants seized the army's Regiment 121 at Maylabieh in the northern Hassakeh province after a three-day battle.

In the past, Islamic State fighters and government forces have largely avoided engaging each other, triggering accusations among mainstream Syrian rebels fighting to topple Assad that the two sides were colluding against them.

Those accusations have been blunted by the recent fighting, which suggests the Islamic State is fighting on all fronts in its quest to expand territory under its control in Syria.

The Britain-based Syrian Observatory for Human Rights said more than 2,000 people have been killed since Assad's inauguration speech on July 16, nearly half of them soldiers and government-allied militiamen.

It didn't provide a breakdown for the rest of the casualties, which would include civilians and opposition fighters.

"These are the highest losses for regime forces suffered in the space of 10 days since the uprising against Assad began" in March 2011, said Rami Abdurrahman, the director of the Observatory. The group documents losses on the opposition and government side through a network of activists on the ground in Syria.

Other activists in Syria confirmed that past weeks have seen a record death toll.

The Britain-based Observatory said in July that 171,000 people have been killed since the conflict began in March 2011. At that time, it said the dead included 39,036 government forces, 24,655 pro-government gunmen, 15,422 opposition fighters, 2,354 army defectors and more than 500 Lebanese fighters from the Hezbollah militant group that is backing Assad. The rest were mostly civilians.

However, the Syrian government has not reported on the heavy losses.

Beyond Syria, the Islamic State fighters have also seized large swaths of land in northern and western Iraq in a blitz offensive that began last month, and have declared a self-styled caliphate across the territory they now control, straddling the Iraq-Syria border.