The Pakistani-based group opposes the government in Tehran and has in the past carried out successful attacks against Iranian troops, including kidnappings, and an April 2017 ambush on the Pakistan border that killed nine border guards.
In a separate statement before leaving for the Russian city of Sochi, Rouhani was also quoted by the IRNA news agency as saying that Tehran will make those responsible for the deadly incident "pay for the blood of our martyrs".
Number of the killed may increase.
He didn't specify those countries, but Tehran has often accused UAE and Saudi Arabia in the past of supporting Sunni militant groups inside Iran, an accusation both countries have denied.
Earlier, Iranian Foreign Minister Mohammad Javad Zarif said that "it is no coincidence" that his country was hit on the same day that the U.S. held a summit in Poland, which is seen as aimed at countering Iran.
Jaish al-Adl announced on the messaging app Telegram that it carried out the attack.
The force said on Wednesday a suicide bomber driving a vehicle laden with explosives had attacked a bus transporting members of the Guards in the province of Sistan-Baluchestan.
Though a Sunni Muslim militant group-Jaish al Adl, meaning Army of Justice-claimed responsibility for the attack, Rouhani said the influence of foreign actors was clear, Reuters reported. "Especially when cohorts of same terrorists cheer it from Warsaw streets & support it with twitter bots?" "United States seems to always make the same wrong choices, but expect different results".
A senior Revolutionary Guards commander said on Wednesday that Iran response to a suicide attack that killed 27 soldiers in southeastern region will not be limited to Iranian borders.
On January 29, three members of an Iranian bomb squad sent to the scene of an explosion in provincial capital Zahedan were wounded when a second device blew up as they were trying to defuse it.
Also in December of the same year, four Iranian policemen were killed and 42 other people were wounded in a suicide auto bomb attack on a police headquarters in the country's southeast.
Another of the bloodiest attacks in recent times to have hit Iran happened in September when assailants killed 24 people at a military parade in the southwestern city of Ahvaz.