US President Barack Obama has called the apparent gassing of hundreds of Syrian civilians a "big event of grave concern".

However, in an interview with CNN he stressed he was in no rush to send in US troops.

Opposition activists said between 500 and 1,000 were killed and more bodies were being found in the wake of Wednesday's incident.

The Syrian government has insisted it was not responsible.

Mr Obama said: "Sometimes what we've seen is that folks will call for immediate action, jumping into stuff that does not turn out well, gets us mired in very difficult situations, can result in us being drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region.

"The United States continues to be the one country that people expect can do more than just simply protect their borders.

"But that does not mean that we have to get involved with everything immediately. We have to think through strategically what's going to be in our long-term national interests."

Asked about his comment that chemical weapons would be a "red line" for the US, he replied: "If the US goes in and attacks another country without a UN mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it."

A White House spokesman also reiterated Mr Obama's position that he did not expect to have "boots on the ground" in Syria.

US assistance to Syrian opposition fighters was on an "upward trajectory" that was expanding in scope and scale, the spokesman said.

Mr Obama's caution contrasted with calls for action from NATO allies, including France, Britain and Turkey.

Those countries see little doubt President Bashar al-Assad's forces staged pre-dawn missile strikes on Wednesday.

International powers, including Russia, have urged Mr Assad to cooperate with the UN inspection team that arrived on Sunday to pursue earlier allegations of chemical weapons attacks.

However, there has been no public response from the Syrian government.

Russian Foreign Minister Sergei Lavrov spoke to US Secretary of State John Kerry by phone yesterday and they had agreed that an objective investigation was needed.

However, Russia has also said rebels may have released gas to discredit Mr Assad and they were impeding an investigation.

UN Secretary-General Ban Ki-moon said he intends to conduct a "thorough, impartial and prompt investigation" into the allegations.

Top UN disarmament official Angela Kane is due to arrive in Damascus on Saturday to push for access to the site for the UN inspectors.

"I can think of no good reason why any party - either government or opposition forces - would decline this opportunity to get to the truth of the matter," Mr Ban said.

Activists take samples from victims

Activists say they have prepared body tissue samples from victims of the attack near Damascus and are trying to get them to the team of UN inspectors staying in a hotel a few kilometres away.

"The UN team spoke with us and since then we prepared samples of hair, skin and blood and smuggled them back into Damascus with trusted couriers," said activist Abu Nidal, speaking from the rebel-held town of Arbin.

Several activists in the area who spoke to Reuters said they too had prepared samples to smuggle into the capital, but were unable to find a way to access the monitors inside their hotel.

Images, including some by freelance photographers supplied to Reuters, showed scores of bodies laid out on floors with no visible signs of injury. Some had foam at the nose and mouth.

UN says one million children have fled Syria

Meanwhile, the United Nations has estimated that the number of Syrian children forced to flee their country has reached one million.

The UN's refugee agency and its Children's Fund said that another two million Syrian minors are uprooted within their country and are often attacked or recruited as fighters in violation of humanitarian law.

They described it as a shameful milestone and said they are struggling to meet the needs of the refugees.

Head of the UN Refugee Agency Antonio Guterres said many of the children were showing signs of trauma, including loss of speech and behavioural disorders.

Speaking at a news conference in Geneva, Mr Guterres said: "One million children is more than the number of children living in Wales.

"One million children is more than the number of children living in Los Angeles and Boston combined.

"Can you imagine Wales without children? Can you imagine Boston and Los Angeles without children?"

He said there was an "enormous risk of Syria facing the problem of a lost generation".