It's time for the United States to hold Iran accountable.

Iran has arrested almost 50 people in connection with twin attacks on Tehran that killed 17 people last week, officials said, as security forces stepped up efforts to crack down on suspected militants.

Islamic State claimed responsibility for the suicide bombings and gun attacks on parliament and the mausoleum of the Islamic Republic's founder, Ayatollah Ruhollah Khomeini, on Wednesday.

Iran's intelligence minister Mahmoud Alavi said on Saturday night that 43 suspects had been arrested and operations to identify and crush more "terrorists cells" were under way.

On Sunday, the head of the justice department in Kordestan province in western Iran announced more arrests.

"Six people who were certainly connected to Wednesday's terrorist attacks in Tehran were identified and arrested," Aliakbar Garousi was quoted as saying by the Fars news agency.

Iran also said its security forces killed the mastermind of the attacks on Saturday.

"The commander of this terrorist group was based in the border regions, but after the attacks left the country," Alavi was quoted as saying by state broadcaster IRIB on Sunday.

"However, with cooperation of (intelligence) services that are close to Iran, he paid the price of his crimes on Saturday and was killed by Iran's security forces and our friends in the other intelligence services," he said.

Iran's intelligence ministry said on Thursday that five of the gunmen and bombers were Iranian members of Islamic State who had fought in the militants' strongholds in Syria and Iraq.

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