BEIRUT (Reuters) - Islamic State has bolstered its forces north of Syria's second city Aleppo, where it is attacking rivals as part of a broader push beyond its eastern strongholds, a rebel leader and a monitoring group said.
The radical jihadist group has in recent weeks conducted a series of attacks in western areas of Syria including state-held territory and has also staged a major advance into Damascus, where it is battling for control of the Yarmouk refugee camp.
Earlier this week, the group targeted rival rebel factions north of Aleppo with two car bombs, killing at least 31 people, according to the Syrian Observatory for Human Rights, which monitors the conflict.
A rebel commander with fighters in that area said Islamic State had called in reinforcements. "They increased their presence and they are launching an attack," said the rebel who declined to be named, citing the safety of his fighters.
"They are sending messages to terrorize the people, spreading rumors among the people that they are coming back."
The Observatory on Friday reported fighting in a town near the site of the double car bombing.
"Daesh is trying to target the Islamist and (other) fighting factions in the northern Aleppo countryside," said Rami Abdulrahman, who runs the British-based Observatory. Daesh is an Arabic name for Islamic State used by its opponents.
"They are trying to take the initiative."
Aleppo itself remains divided between an area held by the government and another held by an array of insurgent groups.
The latest drive follows attacks by Islamic State fighters further south in government-held areas, including Hama province where last week the group massacred some 40 people during an assault on a village.
The Observatory said on Friday that Islamic State was still holding 50 people seized in the raid.
The group's capture of Yarmouk, still the focus of fierce battles, would give it a foothold just a few kilometers from the palace of Syrian President Bashar al-Assad.
U.N. Secretary General Ban Ki-moon described Yarmouk, estimated to be home to 18,000 people, as the "deepest circle of hell" and "a death camp" on Thursday. He said the residents were being "held hostage" by Islamic State and other extremists.
As recently as February, Islamic State had withdrawn forces and hardware from Aleppo province in a redeployment seen linked to the pressure it was under elsewhere in Syria.
The group, which has declared a cross-border "caliphate" in Iraq and Syria, has faced U.S.-led air strikes and attacks by Kurdish militia in the northeast.
As Islamic State attempts to expand west, it may become more complicated for the U.S.-led alliance to press the aerial campaign. The Syrian military last month shot down a U.S. drone that was flying over an area of western Syria.
Islamic State remains the single most powerful insurgent group in the country, where most of the territory it holds has been seized from weaker anti-Assad factions.
Its move into Yarmouk last week was assisted by fighters from the al Qaeda-linked Nusra Front, showing the scope for cooperation between groups that have fought each other elsewhere and continue to do so, notably in the north.
The group continues to battle rival insurgents for control of Yarmouk camp, where the Syrian military has been mounting air strikes, the Observatory has reported.
On Friday, it reported overnight fighting between government forces backed by a Palestinian faction and "Islamic factions".
Writing by Tom Perry; Editing by Crispian Balmer