The United Kingdom reopened its embassy in Iran Sunday, after the site was evacuated by British diplomatic staff more than four years ago.

"Today's ceremony marks the end of one phase in the relationship between our two countries and the start of a new one – one that I believe offers the promise of better," said U.K. Foreign Secretary Philip Hammond.

In an optimistic speech given at the embassy's reopening ceremony in Tehran, Hammond said since President Hassan Rouhani was elected in 2013, ties between the U.K. and Iran had recovered “step by step.”

"In the first instance, we will want to ensure that the nuclear agreement is a success, including by encouraging trade and investment once sanctions are lifted," he said, referring to a recent Iranian deal with world powers over its nuclear program.

Iran also reopened its embassy in London, hours after the two countries restored diplomatic relations.

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Hammond's visit is the first time a British foreign secretary has visited Iran since 2003, and only the second since the U.S.-backed shah was overthrown in the 1973 Islamic Revolution. The embassy itself has been closed since November 2011, when protesters stormed the building amid protests against an escalation in U.K. sanctions.

After two years of stagnant diplomatic ties, in late 2013 the two countries took their first step towards repairing relations by exchanging non-resident chargés d’affaires.

Hammond is now set to hold high level talks with Iranian officials, aimed at boosting bilateral ties.

According to IranianstatenewsagencyIRNA, “He is to conferwithrankingIranianofficialsincluding … Rouhani, ParliamentSpeaker Ali Larijani as well as his IraniancounterpartMohammadJavad Zarif.”

“He will also meet with Secretary of the Supreme National Security Council Ali Shamkhani, Minister of Petroleum Bijan Namdar Zangeneh and Governor of the Central Bank of Iran Valiollah Seif,” IRNA reported.