Gunmen attacked Iranian President Mahmoud Ahmadinejad's election campaign centre in the southeastern city of Zahedan on Friday, a day after a deadly mosque bombing in the city blamed on Washington.
State-run IRNA news agency said gunmen on motorbikes opened fire at the centre, wounding three people including a child who needed surgery for stomach wounds.
The early evening attack came a day after a suicide bomber killed 25 people and wounded 125 others in an attack on a Shiite mosque in Zahedan, restive capital of Sistan-Baluchestan province bordering Afghanistan and Pakistan.
Sistan-Baluchestan deputy governor Jalal Sayah told the semi-official Fars news agency that the bombers of the Shiite Amir al-Momenin mosque were "hired by America and the agents of the arrogance" -- a reference to Washington.
But Washington rejected the claim. "We condemn this attack in the strongest possible terms," State Department spokesman Ian Kelly said.
"We do not sponsor any form of terrorism in Iran and we continue to work with the international community to try to prevent any attacks against innocent civilians anywhere," he said.
"We note with concern a recent trend of bombings of Shiite mosques in Iraq and Pakistan as well as in Iran and strongly condemn any kind of sectarian driven violence," Kelly said.
Iran's state-broadcaster said the pan-Arab television channel Al-Arabiya reported that the Jundullah (Soldiers of God) Sunni rebel group said it was behind the mosque attack.
"Three knife-wielding people on motorbikes stopped outside the centre on Saadi Street, cursed, made threats and tore up billboards," Mohammad Reza Zahed Sheikhi, who heads the election office, told IRNA of Friday's shooting.
When campaign workers for the June 12 presidential election went to protest, "the attackers pulled out guns and shot at them," he said, adding that two campaign workers and a child were wounded.
"The men ran away, but the police chased and arrested them."
A senior government official who declined to be named told AFP incidents targeting election candidates will be probed.
The mosque and election office attacks were reminiscent of violence that preceded Iran's last presidential election in 2005 which Ahmadinejad won, when at least eight people were killed in bombs that hit Tehran and the southwestern city of Ahvaz.
Interior Minister Sadegh Mahsooli on Friday also pointed the finger at the United States and Israel.
"The terror agents are neither Sunni nor Shiite but American and Israeli seeking a Sunni-Shiite divide," Mehr news agency quoted him as saying.
"Enemies try to influence the election by terror, just as they did in Zahedan yesterday."
Iran's supreme leader Ayatollah Ali Khamenei urged restraint, in a statement read on state television.
Sunni clerics "should once again express their firm stand in loathing the corrupt ones who commit such crimes in the name of defending Sunni adherents. Shiite clerics should prevent thoughtless and angry reactions," he said.
"Those who committed this enormous sin may have done it out of anger and ignorance, but undoubtedly the hands of political planners in some interfering powers and their spy services are covered in the innocent victims' blood."
Zahedan MP Payman Foroozesh told ILNA news agency the mosque attacker was a suicide bomber.
Provincial justice chief Ebrahim Hamidi told ISNA news agency that one person had been arrested over the bombing and "charged with armed opposition and acting against national security."
Former Iranian premier and presidential hopeful Mir Hossein Mousavi blamed "foreign forces" for the mosque blast.
"The fewer foreign forces in the region, the more security there is. They provoke extremism in the region such as the incident in Zahedan," said Mousavi, one of four candidates in the presidential election.
Iran has often blamed US and British agents based in neighbouring Iraq and Afghanistan for attacks in border provinces with significant ethnic minorities.
Sistan-Baluchestan province has a large ethnic Sunni Baluch minority and has seen in recent years a deadly insurgency by Jundullah, which opposes the government of predominantly Shiite Iran.
The province also lies on a major narcotics-smuggling route from Afghanistan and Pakistan.