BAGHDAD (AP) – A rocket landed near the prime minister’s office Thursday during the first visit to Iraq by the head of the United Nations in nearly a year and a half, sending Secretary-General Ban Ki-moon ducking unharmed behind a podium at a news conference.

The attack came as Prime Minister Nouri al-Maliki’s government said it had been negotiating with Sunni insurgents for months, and the American military said that it had released a senior aide to Shiite cleric Muqtada al-Sadr on al-Maliki’s request.

The rocket caused no injuries but rattled the building in the heavily guarded Green Zone, sent small chips of debris floating from the ceiling, and left a three-foot-wide crater about 50 yards away outside.

It struck right after Mr. al-Maliki, standing next to Mr. Ban, had finished telling reporters that Ban’s visit was a sign that Iraq was on the road to stability.

“We consider it a positive message to (the) world in which you confirm that Baghdad has returned to playing host to important world figures because it has made huge strides on the road toward stability,” al-Maliki said in his opening remarks.

Mr. Ban had just finished giving an answer to question and it was being translated into Arabic as the rocket struck with a big explosion.

He appeared frightened, casting his eyes right and left as he rose after ducking behind the podium where he was standing and answering questions with Mr. al-Maliki. A worried-looking Mr. Ban turned to one of his aides and asked: “Is it OK?”

Mr. al-Maliki told his security guards, “Nothing’s wrong,” as one of them moved to grab him. He then proceeded to answer a question and while that response was being translated, he turned to Ban and asked: “That’s enough?”

“Yes,” he replied.

The last visit to Iraq by the head of the U.N. was in November 2005, by Mr. Ban’s predecessor, Kofi Annan.

The United Nations headquarters in Baghdad was bombed by militants on Aug. 19, 2003, and 22 people died, including the top U.N. envoy, Sergio Vieira de Mello. The U.N.’s international staff withdrew from Iraq in October 2003 following a second assault on its offices and other attacks on humanitarian workers. A small staff has gradually been allowed to return since August 2004.

America said it believed that the peace process could be helped by Ahmed al-Shibani, who was captured in the holy Shiite city of Najaf during fierce clashes in 2004 between U.S. forces and al-Sadr’s Mahdi Army militia, which has largely cooperated with a new security push by U.S. and Iraqi forces.

The government released a photograph showing Mr. al-Maliki receiving a smiling Mr. al-Shibani at his office, underlining the close ties between the prime minister and Mr. al-Sadr. Mr. al-Sadr’s support kept Mr. al-Maliki in his job last year, since the cleric’s loyalists have 30 of parliament’s 275 seats and six Cabinet posts.

The American military said it had determined that Mr. al-Shibani “could play a potentially important role in helping to moderate extremism and foster reconciliation in Iraq.”

Authorities imposed an indefinite curfew on the southern city of Basra after clashes between the Mahdi Army and the rival Shiite Fadhila party, which recently withdrew from al-Maliki’s government.

The continuing rivalry in the oil-rich region poses the most serious threat to the unity of Iraq’s majority Shiites.

Police said the Mahdi militiamen fought Fadhila guards outside party headquarters, capturing eight before the building caught fire. Twelve Mahdi Army fighters were wounded, they said.

Clashes also erupted near the residence of Basra’s Fadhila governor, Mohammed al-Waeli, and continued into the afternoon, police said.

The clashes came days after British forces pulled out of their main base in the heart of Basra.

The American military announced that it had captured the leaders of a Shiite insurgent network “directly connected” to the killing in January of five American soldiers in the holy city of Karbala by gunmen speaking English, wearing U.S. military uniforms and carrying American weapons.

The military said the arrests of Qais Khazaali, his brother Laith Khazaali and several other members of the network took place over the past three days.

On Wednesday, two senior Mahdi Army commanders told The Associated Press that Qais al-Khazaali was the leader of up to 3,000 fighters who defected from the militia and were now financed directly by Iran and no longer loyal to firebrand cleric Muqtada al-Sadr.

Saad Yousif al-Muttalibi of the Ministry of National Dialogue and Reconciliation told The Associated Press in a telephone interview that talks with Sunni insurgent groups were initiated at the request of the insurgents and have been taking place inside and outside Iraq over the past three months.

He refused to identify the groups, but said they did not include Al Qaeda in Iraq or Saddam Hussein loyalists. Members of the former president’s outlawed Baath Party took part, he added.

Mr. al-Muttalibi said the negotiations were deadlocked over the insurgent groups’ insistence that they would lay down their arms only when a timetable for the withdrawal of American-led coalition troops in Iraq is announced.

The government’s response was that such a move could only be taken when security is restored.

Future rounds of negotiations are planned, he said, but did not elaborate.

The American military reported that two soldiers and a Marine were killed in combat on Wednesday. One soldier was killed in Baghdad; a second soldier and a Marine perished in Anbar province, the Sunni insurgent stronghold west of Baghdad.

The clashes in Basra, 340 miles southeast of Baghdad, followed the arrest of a Mahdi Army militiaman by policemen thought to be loyal to Fadhila, and the retaliatory kidnapping and killing of a Fadhila supporter, according to police.

In the volatile city of Baqouba northeast of Baghdad, the bullet-ridden body of a kidnapped local official and mother of three was found dumped on a city street, one day after masked gunmen stormed her house and took her away handcuffed, police said.

Ilham Namik Shahin, 43, was a Shiite member of the Baqouba provincial council. Her brother, Najah Namik Shahin, said 10 gunmen stormed the family home Wednesday night, ordered everyone into the living room before they handcuffed his sister and left with her.

“It took just 10 minutes and we were really scared. We couldn’t talk,” said the brother.

Baqouba, in Diyala province 35 miles northeast of Baghdad, has seen an upsurge in violence and sectarian killings in recent weeks, with Sunni insurgents loyal to al-Qaida in Iraq stepping up attacks as violence appears to ebb in Baghdad since the security push began Feb.14.

Also in Basra, police said gunmen on a motorcycle shot dead a postgraduate female student at Basra University, Tuhfa Jaafar al-Bachay, Wednesday night outside her home.

Like most killings that take place daily in Iraq, the motive for al-Bachay’s murder was unknown.

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Associated Press writers Sameer N. Yacoub and Sinan Salaheddin contributed to this report.