The whereabouts of two bishops kidnapped in northern Syria remain unknown, Syrian church officials said Wednesday, a day after telling reporters that they had been released.

Bishop Tony Yazigi of the Damascus-based Greek Orthodox Church said Tuesday that the bishops, both working in the northern city of Aleppo, had been released. But later on Tuesday, the Syrian Orthodox Patriarchate in the capital said in a statement on its website that it had not received "any official document indicating the (bishops') release."

Gunmen pulled Bishop Boulos Yazigi of the Greek Orthodox Church and Bishop John Ibrahim of the Assyrian Orthodox Church from their car and killed their driver on Monday while they were traveling outside Aleppo. It was not clear who abducted the priests and who is holding them.

But Bishop Tony Yazigi, who is related to one of the abductees, said the gunmen are believed to be Chechen fighters from the al-Qaida-affiliated Jabhat al-Nusra group, one of the most powerful of the myriad of rebel factions fighting in Syria to overthrow the regime of President Bashar Assad. Yazigi declined to say what made it appear that Nusra Front was involved.

The main Syrian opposition group, the Syrian National Coalition, condemned the kidnapping and blamed Assad's regime.

However, the Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, reported that foreign fighters had abducted the bishops near a checkpoint near Aleppo. The Observatory's chief, Rami Abdul-Rahman, said Wednesday that activists in the area where the bishops were kidnapped say the gunmen were foreign fighters from the Caucuses.

Pope Francis called for the rapid release of two bishops. In his appeal Tuesday, the pope called the abduction "a dramatic confirmation of the tragic situation in which the Syrian population and its Christian community is living."

There has been a spike in kidnappings in northern Syria and around Damascus in the past months. Residents blame criminal groups that have ties to both the regime and the rebels for the abductions of wealthy residents traveling to Syria from neighboring Turkey and Lebanon.

Opposition forces control large areas of land in the north and control whole districts inside Aleppo, Syria largest city.

The government still holds large parts of the northern city and its forces daily clash with the opposition fighters, who also control several border crossings with Turkey.

Syrian conflict began in March 2011 as peaceful uprising against Assad's rule. It turned into civil war after some opposition supporters took up arms to fight a harsh government crackdown on dissent.

More than 70,000 people have been killed in fighting, the United Nations says. Nearly 5 million Syrians fled their homes, seeking shelter in neighboring countries or in other parts of Syria where fighting has temporarily subsided.