Islamic State crisis: US-led Syria strikes 'kill 553'
US-led air strikes against Islamic State (IS) militants in Syria have killed 553 people since September, UK-based activists say.
The Syrian Observatory for Human Rights says 464 IS fighters, 57 other militants and 32 civilians have died.
Meanwhile, the US has carried out more air strikes against IS positions near the besieged town of Kobane.
Kurdish sources told BBC Arabic that IS had retaken a strategically important hilltop 4km west of the town.
Kurdish forces fighting IS in Kobane took Tal Shair nine days ago.
Reports also suggest IS fighters have been shelling Kurdish positions and attacking the town centre.
Syrian Kurdish sources told the BBC that IS fighters launched intensive attacks on Wednesday night from the southern and eastern parts of the town, in an apparent attempt to get access to the town centre.
The fighting resumed on Thursday morning after a brief period of calm, the sources added.
The US military says that its fighters and bombers conducted six air strikes against IS positions in Syria on Wednesday and Thursday, including four attacks near Kobane which destroyed IS fighting positions, an IS vehicle and an IS command-and-control centre.
A US military statement said that the attacks in Syria - as well as nine air strikes against IS positions in Iraq on Wednesday and Thursday - were intended to eliminate the "terrorist group" and the threat it poses to the region and the wider international community.
Control of Tal Shair has flipped between Syrian Kurdish fighters and IS militants in recent days. Kurds last captured the hill - close to the border with Turkey - from IS on 14 October.
Analysts say whoever controls the hill has a much better chance of controlling Kobane, a Syrian Kurdish town right up against the border.
Seizing Kobane would give IS full control of a long stretch of Syrian territory along the border.
Kurdish fighters and their Free Syrian Army allies told the BBC that they had launched a counter-attack against IS west of Kobane in an attempt to decrease the pressure on fellow fighters south and east of the town.
Omar Alloush, a Kurdish official in Kobane, told the BBC over the telephone that "the fighting is the heaviest in couple of weeks".
Separately the US envoy tasked by President Obama with building a coalition against IS told the BBC that Turkey - which has been criticised over its inaction regarding Kobane - has done "quite a lot" to help deal with the crisis in Syria.
Gen John Allen said that Turkey had accepted the "enormous burden" of 1.5 million refugees. He said that the US was in discussions with the Turkish government which will result in "other commitments".
Gen Allen also said that the US was working with other governments to cut off the "self-sufficient" funding of IS by targeting its "illicit oil smuggling", its "trafficking of antiquities" and other "criminal activities".
The IS advance in Syria takes place against the backdrop of the civil war there.
US-led air strikes are being conducted without the explicit permission of President Bashar al-Assad, who the West wants to relinquish power.
- Formed out of al-Qaeda in Iraq (AQI) in 2013, IS first captured Raqqa in eastern Syria
- It captured broad swathes of Iraq in June, including Mosul, and declared a "caliphate" in areas it controls in Syria and Iraq
- Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics
- Known for its brutal tactics, including beheadings of soldiers, journalists and aid workers
- The CIA says the group could have as many as 31,000 fighters in Iraq and Syria