The U.N. nuclear agency says Iran must address concerns about its suspected atomic bomb research, as European Union foreign policy chief Catherine Ashton and Iran's chief nuclear negotiator Saeed Jalili prepared to meet in Istanbul later Tuesday.

Ashton says the meeting is “part of continuing efforts to engage with Iran.” A spokesman for the Iranian Foreign Ministry Tuesday urged Western powers to engage in “purposeful negotiations.”

The developments come as Tehran deployed a Russian-made submarine and a destroyer in the Persian Gulf just days after the United States and more than two dozen allies began naval exercises nearby to practice keeping oil shipping lanes open.

Iran has repeatedly threatened to close the Strait of Hormuz, a route for oil exports from the Gulf, if Iranian nuclear sites are attacked by Israel, which believes Tehran is trying to develop an atomic bomb.

Talks between world powers and Iran in Moscow in June failed to reach a breakthrough on the nuclear dispute.

Iran is under international sanctions for its controversial nuclear program. Iran says its program is peaceful while Western nations, Israel and U.N. nuclear experts have expressed fears that Iran is working towards development of a nuclear weapon.

On Monday, Tehran alleged that “terrorists” had infiltrated the U.N.'s International Atomic Energy Agency to sabotage the Islamic Republic's uranium enrichment plants.

The head of Iran's atomic agency, Fereydoun Abbasi-Davani, said power lines to its primary underground nuclear facility at Fordo were cut last month by explosive blasts set off by “terrorists and saboteurs.”

Iran has in the past blamed the United States and Israel for targeting its nuclear program and scientists.

Increased uranium enrichment activity at the Fordo site has raised concerns because it can be a step in weapons development. Iran says the increased enrichment is for medical and energy needs.