The Britain-based Syrian Observatory for Human Rights said the strikes hit mills and grain storage areas in the northern Syrian town of Manbij, in an area controlled by Islamic State, killing at least two civilian workers.
Strikes on a building on a road leading out of the town also killed a number of Islamic State fighters, said Rami Abdulrahman, who runs the Observatory which gathers information from sources in Syria.
The US military said on Monday an American air strike overnight targeted Islamic State vehicles in a staging area adjacent to a grain storage facility near Manbij, but it had no evidence so far of civilian casualties.
While a week of raids has taken a toll on Islamic State equipment and fighters on the ground, there is no sign yet that the tide is being turned against the group, which controls large areas of Iraq and Syria, where it has declared an Islamic Caliphate.
In a statement to the United Nations that appeared to give approval of US and Arab air strikes in Syria against the militants, Syria's foreign minister said his country backed the campaign against Islamic State.
Syria "stands with any international effort aimed at fighting and combating terrorism", Walid al-Moualem said, whose government has long been an international pariah because of what critics say is its brutality in a civil war that has killed 200,000 people.
The US-led strikes have so far failed to halt an advance by fighters in northern Syria on Kobani, a Kurdish town on the border with Turkey where fighting over the past week caused the fastest refugee flight of Syria's three-year-old civil war.
At least 15 Turkish tanks could be seen at the frontier, some with guns pointed towards Syrian territory. More tanks and armoured vehicles moved towards the border after shells landed in Turkey on Sunday and Monday.
ARAB ALLIES
The United States has been bombing Islamic State and other groups in Syria for a week with the help of Arab allies, and hitting targets in neighbouring Iraq since last month. European countries have joined the campaign in Iraq but not in Syria.
Islamic State, a Sunni militant group which broke off from al Qaeda, alarmed the West and the Middle East by sweeping through northern Iraq in June, slaughtering prisoners and ordering Shi'ites and non-Muslims to convert or die.
It is battling Shi'ite backed governments in both Iraq and Syria, as well as other Sunni groups in Syria and Kurdish groups in both countries, part of complex multi-sided civil wars in which nearly every country in the Middle East has a stake.
The head of Syria's al Qaeda branch, the Nusra Front, a Sunni militant group which is a rival of Islamic State and has also been targeted by US strikes, said Islamists would carry out attacks on the West in retaliation for the campaign.
Obama has worked since August to build an international coalition to combat the fighters, describing them last week in an address to the United Nations as a "network of death".
His acknowledgment in an interview broadcast on Sunday that US intelligence had underestimated Islamic State offered an explanation for why Washington appeared to have been taken by surprise when the fighters surged through northern Iraq in June.
The militants had gone underground when US forces quashed al Qaeda in Iraq with the aid of local tribes during the US war there which ended in 2011, Obama told CBS's "60 Minutes".
"But over the past couple of years, during the chaos of the Syrian civil war, where essentially you have huge swathes of the country that are completely ungoverned, they were able to reconstitute themselves and take advantage of that chaos."
BATTLE ON BORDER
Gunfire rang out from across the border and a plume of smoke rose over Kobani as periodic shelling by Islamic State fighters took place. Kurds watching the fighting from the Turkish side of the border said the Syrian Kurdish group, the YPG, was putting up a strong defence.
"Many Islamic State fighters have been killed. They're not taking the bodies with them," said Ayhan, a Turkish Kurd who had spoken by phone with one of his friends fighting with the YPG. He said Kurdish forces had picked up eight Islamic State bodies.
At Mursitpinar, the nearby border crossing, scores of young men were returning to Syria saying they would join the fight. More refugees were fleeing in the opposite direction.
"Because of the bombs, everyone is running away. We heard people have been killed," said Xelil, a 39-year-old engineer who fled Kobani on Monday. "The YPG have got light weapons but Islamic State has big guns and tanks."
A local official in Kobani said Islamic State continued to besiege the town from the east, west and south and that the militants were 10 km (6 miles) from the outskirts.
"From the morning there has been shelling into Kobani and ... maybe about 20 rockets," Idris Nassan, deputy foreign minister in a local Kurdish administration said by phone. He said the rockets had killed at least three people in the town.
Turkey has not permitted its own Kurds to cross to join the battle: "If they've got Syrian identity or passports, they can go. But only Syrians, not Turks," said one Turkish official at the border where security has been tightened.
A NATO member with the most powerful army in the area, Turkey has so far kept out of the US-led coalition, angering many of its own Kurds who say the policy has abandoned their cousins in Syria to the wrath of Islamic State fighters.
GAS PLANT
The Syrian Observatory, which monitors the conflict with a network of sources on the ground, said US-led strikes had hit a Conoco gas plant controlled by Islamic State outside Deir al-Zor city in eastern Syria, wounding several fighters.
The plant feeds a power station in Homs that provides several provinces with electricity and powers oilfield generators, the Observatory said.
The observatory also said warplanes had hit mills and grain storage areas in the northern Syrian town of Manbij, killing civilian workers.
"We are aware of media reports alleging civilian casualties, but have no evidence to corroborate these claims," Colonel Patrick Ryder, a spokesman at the US military's Central Command said, adding the military however took such reports seriously and would look into them further.
US-led warplanes also hit areas of Hasaka city in Syria's north east and the outskirts of Raqqa city in the north, which is Islamic State's stronghold. Syria's state news agency also said US-led forces had carried out strikes in Raqqa province.
