US President Barack Obama has criticized the seizure of British embassy in Tehran, dpa reported.
In a meeting with Prime Minister Mark Rutte of the Netherlands, Obama said, we "are deeply disturbed by the crashing of the English embassy - the embassy of the United Kingdom in Iran. That kind of behavior is not acceptable."
He called on the Iranian government to hold to account those responsible.
"For rioters, essentially, to be able to overrun the embassy and set it on fire is an indication that the Iranian government is not taking its international obligations seriously," Obama said.
A group of Iranian students on Tuesday stormed the British embassy in Tehran, tearing up documents and pictures of Queen Elizabeth II, in the latest instalment of a brewing diplomatic row between the two countries.
Hundreds of students held a protest outside the British embassy, demanding the expulsion of the country's ambassador to Iran.
The demonstration, initially peaceful, changed in tone when some of the students defied anti-riot police stationed at the embassy gate and climbed over the building's wall.
An estimated 10 to 20 students succeeded in getting into the compound, where they started a fire and removed the Union Jack, a policeman at the site said. Several embassy vehicles were also set on fire.
Reports that up to six people had been taken hostage and later freed could not be confirmed.
Such reports raised the spectre of the 1979 hostage crisis, when more than 60 US nationals were taken hostage after Islamist students took over the United States embassy. Some were held for more than a year.
Those who stayed outside the embassy on Tuesday threw stones at its windows and shouted "Death to England" and "Get away England."
British Prime Minister David Cameron condemned the Iranian government's failure to protect the embassy, and said the government would consider what measures to take in the next few days.
"The attack on the British Embassy in Tehran today was outrageous and indefensible. The failure of the Iranian government to defend British staff and property was a disgrace," said Cameron, after an emergency meeting of a government security committee over the issue.
"We hold the Iranian government responsible for its unacceptable failure to protect diplomats in line with international law."
Britain also summoned the Iranian charge d'affaires over the storming, Foreign Secretary William Hague said in a statement.
Hague said his Iranian counterpart had apologized, but added: "This remains a very serious failure by the Iranian government."
All British embassy staff and their dependents in Tehran had been accounted for, according to the most recent information, Hague added.
The Iranian Foreign Ministry distanced itself from Tuesday's incident.
"The Foreign Ministry deplores the unacceptable actions of some of the protesters, which happened despite efforts by police and special forces to prevent them," the ministry said in a press statement.
"We have always respected international rules and regulations with regards to the immunity of diplomatic missions and therefore started immediate investigations of the incident through legal channels," the statement added.
A group of students later gathered outside the residences of the British embassy's diplomats and staff, in the north of the Iranian capital, ISNA news agency reported.
Some of them succeeded in entering the compound, with one of them quoted as saying: "The English spies should leave the country so that we get rid of them."
Police and riot police were stationed at the site, while the Foreign Office in London urged all British nationals in Iran to "stay inside and keep a low profile."
The protest was shown live on Iranian state television.
It followed a parliamentary bill approved on Sunday urging the government to expel the British ambassador and downgrade bilateral ties to charge d'affaires level.
The diplomatic crisis between the two countries was sparked by a recent report by the International Atomic Energy Agency that accused Iran of having tested designs for a nuclear warhead.
The European Union also condemned the incident in Tehran, with EU foreign affairs chief Catherine Ashton calling on the Iranian government to fulfil its international obligations in line with the Vienna Convention to protect diplomats and embassies.
While condemning the storming as violation of international laws, German Foreign Minister Guido Westerwelle ordered the summoning of the Iranian ambassador to Berlin.
French Foreign Minister Alain Juppe also condemned Tuesday's incident.
"Once more the Iranian regime has provided proof of the little consideration it has for international law," Juppe said in a statement.
"In these unacceptable circumstances, France shows complete solidarity with Britain," said Juppe, for whom the attack was a "flagrant and scandalous violation of the Vienna Convention."