The U.N. chief has told Israel the international standoff over Iran's suspect nuclear program must be resolved peacefully.

Secretary-General Ban Ki-moon delivered his message at a time of growing concerns that Israel could be planning an attack on Iran. Israel, like the West, believes Iran is developing nuclear weapons and says no option, including force, can be ruled out in stopping it. Iran insists its nuclear program is peaceful

At a news conference with Israel's prime minister, Ban urged the Iranians to prove their nuclear program is for peaceful purposes. When asked whether he fears an Israeli attack, he said "there is no alternative to a peaceful resolution of these issues."

U.N. nuclear inspectors said Wednesday they would soon return to Tehran after a "good trip" to the Iranian capital.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

VIENNA (AP) _ A senior U.N. nuclear expert on Wednesday announced plans to revisit Tehran soon after a "good trip," indicating progress on his team's quest to probe suspicions that the Islamic Republic is secretly working on an atomic arms program.

The announcement from mission leader Herman Nackaerts came shortly after his group landed at Vienna airport after three days of discussions with Iranian officials in Tehran.

While Nackaerts gave no details on what the International Atomic Energy Agency experts had achieved, diplomats had said before their departure that their main focus was to break Iranian resistance to talking about the weapons program allegations.

"We had three days of intensive discussions about all our priorities, and we are committed to resolve all the outstanding issues," Nackaerts told reporters. "And the Iranians said they are committed, too.

"But, of course, there's still a lot of work to be done," he said. "So we have planned another trip in the very near future."

Asked how the visit was, he replied, "we had a good trip."

Diplomats familiar with IAEA strategy told The Associated Press before the trip that the agency delegation was unlikely to settle for vague promises or complicated plans that could further stall their probe. Nackaerts' announcement that a new visit would soon take place _ plus his praise of a "good trip" _ therefore suggested that he was coming back to Vienna with some progress made.

Any progress on the issue would be significant. Iran has refused to discuss the alleged weapons experiments for more than three years, saying they are based on "fabricated documents" provided by a "few arrogant countries" _ a phrase authorities in Iran often use to refer to the United States and its allies.

Faced with Iranian stonewalling, the IAEA summarized its body of information in November in a 13-page document drawing on 1,000 pages of intelligence. It stated then for the first time that some of the alleged experiments can have no other purpose than developing nuclear weapons.

The IAEA team was seeking progress on its efforts to talk to key Iranian scientists suspected of working on a weapons program. They also hoped to break down opposition to their plans to inspect documents related to nuclear work and secure commitments from Iranian authorities to allow future visits.

Beyond concerns about the purported weapons work, the United States and its allies want Iran to halt uranium enrichment, which they believe could eventually lead to weapons-grade material and the production of nuclear weapons. Iran says its program is for peaceful purposes _ generating electricity and producing medical radioisotopes to treat cancer patients.

Since the discovery in 2002 that Iran was secretly working on uranium enrichment, the nation has expanded that operation to the point where it has thousands of centrifuges churning out enriched material _ the potential source of both nuclear fuel and fissile warhead material.

Iran also has started producing uranium at a higher level than its main stockpile _ a move that would jump start the creation of highly enriched, weapons grade uranium, should it chose to go that route. And it is moving its higher-enriched operation into an underground bunker that it says is safe from attack.

Tehran is under four sets of U.N. Security Council sanctions because of its refusal to heed international concerns about its nuclear programs, as well as penalties imposed by the United States and Western nations meant to force it into dialogue.

The European Union last week imposed an oil embargo on Iran and froze the assets of its central bank. In December, the United States said it would bar financial institutions from the U.S. market if they do business with Iran's central bank.

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AP video reporter Philipp Jenne contributed.