* Syria says US intervention would be "no picnic"
* Iran warns of severe consequences
* Syria suggests access will not be granted to chemical attack site (Releads with Syria saying it will allow inspection, U.N. and U.S. comments)
By Oliver Holmes
BEIRUT, Aug 25 (Reuters) - Syria agreed to allow the United Nations to inspect the site of a suspected chemical weapons attack from Monday but a U.S. official said it was already too late.
Foreign powers have been searching for a response since many hundreds of people were killed by poisonous gas on Wednesday in the suburbs of Damascus in what appears to have been the world's worst chemical weapons attack in 25 years.
The United Nations said Damascus had agreed to a ceasefire while a U.N. team of experts are at the site for inspections which will begin on Monday. Syria confirmed it had agreed to allow the inspections.
But there were increasing signs that the United States and its allies were considering taking action, a year after President Barack Obama said the use of chemical weapons was a "red line" that would prompt serious consequences.
A senior U.S. official said there was very little doubt that the Syrian government had used a chemical weapon against civilians on Wednesday and that Washington was still weighing how to respond.
The official also said any decision to grant access to the U.N. inspectors would be "too late to be credible" because evidence had been corrupted by government shelling and other actions.
Syria's information minister said any U.S. military action would "create a ball of fire that will inflame the Middle East". He also said Damascus had evidence that chemical weapons were used by rebels fighting to topple President Bashar al-Assad, not by his government. Western countries say they believe the rebels do not have access to poison gas.
A team of U.N. chemical weapons inspectors had already arrived in Syria three days before Wednesday's incident to investigate other earlier reports of chemical weapons use.
Since Wednesday, the 20-strong team has been waiting in a Damascus luxury hotel a few miles from the site of what appears to have been the world's worst chemical weapons attack since Saddam Hussein's forces gassed thousands of Iraqi Kurds in 1988.
Opposition activists in Damascus said the army was using surface-to-surface missiles and artillery to strike eastern Damascus on Sunday, including neighbourhoods where the mass poisoning occurred.
U.S. President Barack Obama met his top military and national security advisers on Saturday to debate options. U.S. naval forces have been repositioned in the Mediterranean to give Obama the option of an armed strike.
"Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts gathered by open sources, the U.S. intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident," the senior U.S. official said.
President Bashar al-Assad's closest ally Iran, repeating Obama's own previous rhetoric, said the United States should not cross a "red line" by attacking Syria.
"America knows the limitation of the red line of the Syrian front and any crossing of Syria's red line will have severe consequences for the White House," said Massoud Jazayeri, deputy chief of staff of Iran's military, Fars news agency reported.
Two and a half years since the start of a war that has already killed more than 100,000 people, the United States and its allies have yet to take direct action, despite long ago saying Assad must be removed from power.
After concluding that Assad's forces had already used a small amount of nerve gas, Obama authorised sending U.S. weapons to Syrian rebels in June. But those shipments were delayed due to fears that radical Sunni Islamist groups in the opposition could gain further ground in Syria and become a threat to the West.
Stark video footage from Wednesday's apparent attack - which showed bodies of people stacked up in medical clinics - has stoked demands abroad for a robust, U.S.-led response after 2-1/2 years of international inaction on Syria's conflict.
The Syrian opposition says between 500 and well over 1,000 civilians were killed this week by gas in munitions fired by pro-government forces. The medical charity Medecins Sans Frontieres said three hospitals near Damascus had reported 355 deaths in the space of three hours out of about 3,600 admissions with nerve gas-type symptoms.
About 60 percent of Americans surveyed in a Reuters/Ipsos poll published on Saturday said the United Nations should not intervene, while just 9 percent thought Obama should act.
The head of the al Qaeda-linked Nusra Front rebel group has pledged to target communities from Assad's Alawite sect with rockets in revenge for Wednesday's incident, according to an audio recording published on YouTube.
"For every chemical rocket that had fallen on our people in Damascus, one of their villages will, by the will of God, pay for it," Abu Mohammad al-Golani said in the recording. (Additional reporting by Khaled Yacoub Oweis in Amman, Mahmoud Habboush in Dubai Yeganeh Torbati in Dubai; Editing by Philippa Fletcher and Peter Graff)