"The leaders discussed the strong partnership and excellent counterterrorism cooperation between the United States and Bulgaria, and the President expressed his appreciation for Bulgaria’s important contributions as a NATO member to international peace and security," according to a readout of the president's call provided to the media.

The suicide bomber has not yet been identified, although Bulgarian Interior Minister Tsvetan Tsvetanov said he was approximately 36 years old and carried a fake driver's license from Michigan. Israeli Prime Minister Benjamin Netanyahu has suggested that Hezbollah, and by extension Iran, was responsible for the attack, a charge the Iranian government has denied.

On Wednesday, the president issued a statement condemning the attacks.

"These attacks against innocent civilians, including children, are completely outrageous," Obama said. "The United States will stand with our allies, and provide whatever assistance is necessary to identify and bring to justice the perpetrators of this attack."

Republican presidential candidate Mitt Romney also issued a statement of condolences on Wednesday.

“The terrorist attack today in Bulgaria is a sobering reminder that the scourge of terrorism continues to threaten all free people," Romney said in a statement. "My heart goes out to the victims and their families, and to all Israelis who have been the targets of such brutal and cowardly violence for so long. We must stand together in the fight against terrorism, and we must prevail.”