TEHRAN - Britain's foreign secretary reopened his country's embassy in Tehran on Sunday in a long-awaited step signalling better relations four years after a mob stormed the compound, forcing its closure.

Philip Hammond's two-day trip comes five weeks after Britain and five other world powers struck a deal with Iran to end a 13-year dispute over the Islamic republic's nuclear programme.

He led a ceremony in the embassy garden with Ajay Sharma, the new charge d'affaires who will represent Britain in Tehran, where the British flag was raised. But the opening did not pass without incident as reporters spotted a "Death to England" slogan still scrawled in red paint on a door at the embassy residence.

Anti-British slogans were also chanted hours later by around 40 protesters outside, 10 of whom were arrested, the official IRNA news agency said, and hundreds of anti-riot police were on duty.

Iran's embassy in London also reopened on Sunday. The two nations are expected to appoint ambassadors within months. Hammond, the first British foreign secretary to visit Iran since Jack Straw in 2003, described the violence in 2011 as "a low point" but said a new journey was beginning.

"Over the coming months, we will work to ensure that the nuclear agreement is a success, including by making sure that it is fully implemented by all sides," he said at the reopening. "Through this embassy's efforts we will support British trade and investment, once sanctions are lifted. That will bring benefits for Britain and the Iranian people."

He later held a press conference with Mohammad Javad Zarif, Iran's foreign minister and lead negotiator in the near two years of nuclear talks that have ended Tehran's isolation from the West.

"We have come a long way but let's walk first and try to run later," Hammond said, again noting that bringing the nuclear deal into force and building confidence was the priority. "It will be a process of evolution to overcome the deficit of trust."

Zarif welcomed Hammond's visit, saying Iran was intent on using dialogue to tackle disagreements and address the troubles of the Middle East in what was a "restart" in relations with Britain.

Meanwhile, Iranian foreign minister said on Sunday it was too early to talk of reopening the US Embassy in Tehran, as Britain restored its diplomatic mission four years after protesters ransacked the British ambassador's residence.

Iran's Foreign Minister Javad Zarif, asked if Tehran would now countenance a restoration of the US diplomatic mission, said the United States' "illogical attitude" towards Iran meant the time was not ripe for a similar move with Washington.

"It seems that there needs to be a change in that kind of attitude and behaviour on the part of the US So the situation is different with the US," he said.

The US embassy was sacked in the early days of the Islamic Revolution in 1979 by students who feared a repeat of a 1953 coup, when the CIA orchestrated the overthrow of Iran's prime minister. The ensuing US hostage crisis lasted 444 days and Washington and Tehran have yet to restore diplomatic ties.

European officials have been quick to visit Iran since July 14, when the nuclear accord with Britain, China, France, Germany, Russia and the United States was announced in Vienna.

The deal will see the lifting of economic sanctions - imposed as punishment over Iran's nuclear programme - in exchange for curbs and a new inspections regime.

Iran's leaders have always denied international allegations that they sought to develop a nuclear weapon, insisting the programme is for peaceful energy and medical purposes.

The thaw between Britain and Iran started with the June 2013 presidential election victory of Hassan Rouhani, a moderate who reached out to the West. Hammond will meet him on Monday.

British Prime Minister David Cameron met Rouhani in New York last autumn, the first such contact at that level since the 1979 Islamic revolution.

Following the 2011 embassy attack, Britain said it could not have happened without the tacit consent of Iran's then regime.

It took place after lawmakers voted to expel Britain's ambassador and reduce trade relations in retaliation for nuclear-related sanctions against Iran's banking sector.

Students rampaged for hours through the embassy compound, tearing down the British flag, ripping up pictures of Queen Elizabeth II and trashing offices. Diplomatic relations were reduced to their lowest possible level, with Britain expelling Iran's officials.

Plans to reopen the embassy were announced in June last year. Sharma was appointed in a non-resident position in November 2013 and has since visited Iran 12 times.

Hammond and treasury minister Damian Hinds are visiting Tehran with a small trade delegation to discuss business opportunities.

Hammond follows his Italian, French and German counterparts who all visited Iran after the nuclear deal. However the United States, which led the nuclear talks, has not had diplomatic relations with Iran since 1979 following the 444-day hostage crisis that followed the storming of its Tehran embassy.

The nuclear deal is undergoing a bruising review in the US Congress, but President Barack Obama has vowed to veto any vote against the historic agreement.