A suicide bombing targeting a bus carrying personnel of Iran's elite paramilitary Revolutionary Guard force killed at least 20 people and wounded 20 in the country's southeast, state media reported. An Al Qaeda-linked Sunni extremist group operating across the border in Pakistan reportedly claimed the assault.

The bombing came on the same day a U.S.-led conference in Warsaw was to include discussions on what America describes as Iran's malign influence across the wider Mideast. It also comes two days after Iran marked the 40th anniversary of its 1979 Islamic Revolution and four decades of tense relations with the West.

Iran's Foreign Minister Mohammad Javad Zarif directly linked the meeting to the attack.

"Is it no coincidence that Iran is hit by terror on the very day that (hashtag)WarsawCircus begins?" Zarif wrote on Twitter. "Especially when cohorts of same terrorists cheer it from Warsaw streets & support it with (Twitter) bots?"

The state-run IRNA news agency, citing what it described as an "informed source," reported the attack on the Guard in Iran's Sistan and Baluchistan province.

The province, which lies on a major opium trafficking route, has seen occasional clashes between Iranian forces and Baluch separatists, as well as drug traffickers.

The Guard is a major economic and military power in Iran, answerable only to the country's supreme leader, Ayatollah Ali Khamenei. It later issued a statement saying a vehicle loaded with explosives targeted a bus carrying border guards affiliated with its force.

While Iran has been enmeshed in the wars engulfing Syria and neighboring Iraq, it largely has avoided the bloodshed plaguing the region. In 2009, more than 40 people, including six Guard commanders, were killed in a suicide attack by Sunni extremists in Sistan and Baluchistan province. Jundallah, a Sunni extremist group still active in the region on Iran's border with Pakistan, claimed responsibility for that attack.

More recently, another Sunni extremist group known as Jaish al-Adl linked to Al Qaeda, kidnapped 11 Iranian border guards in October. Five later were returned to Iran and six remained held.

Both official and semi-official Iranian media outlets blamed Wednesday's bombing on Jaish al-Adl, or "Army of Justice," saying the group had claimed the attack.

That group formed in 2012 and drew some militants from Jundallah, experts believe. Iran long has suspected Saudi Arabia of supporting the militants, something Riyadh denies. It's also unclear how the militants have been able to operate freely from Pakistan for years.

A coordinated June 7, 2017 Islamic State group assault on Parliament and the shrine of Ayatollah Ruhollah Khomeini, the leader of Iran's Islamic Revolution, left at least 18 people killed and more than 50 wounded.

And most recently in September, militants disguised as soldiers opened fire on a military parade in Iran's oil-rich southwestern city of Ahvaz, killing 24 people and wounding over 60. Khamenei blamed Riyadh and Abu Dhabi for the attack, allegations denied by both countries.

Arab separatists in the region claimed responsibility, as did the Islamic State group.

The attacks come as Iranian officials have said they blame Saudi Arabia and the United States for stirring up dissent in the country. President Donald Trump, who campaigned on a promise of tearing up Tehran's nuclear deal with world powers, withdrew the U.S. from the accord last May. Since then, the United Nations says Iran has kept up its side of the bargain, though Iranian officials have increasingly threatened to resume higher enrichment.

Amid the new tensions, Iran's already-weakened economy has been further challenged. There have been sporadic protests in the country as well, incidents applauded by Trump amid Washington's maximalist approach to Tehran.

Khamenei, who earlier approved President Hassan Rouhani's outreach to the West during the nuclear deal negotiations, dismissed any future dealings with the U.S.

"About the United States, the resolution of any issues is not imaginable and negotiations with it will bring nothing but material and spiritual harm," Khamenei said in a statement.

The Warsaw summit, which started Wednesday, was initially pegged to focus entirely on Iran. However, the U.S. subsequently made it about the broader Middle East, to boost participation.

Zarif earlier predicted the Warsaw summit would not be productive for the U.S.

"I believe it's dead on arrival or dead before arrival," he said at a news conference before the bombing.