The state news agency said Wednesday that the union "proves that this opposition was never anything other than a tool used by the West and by terrorists to destroy the Syrian people."
It added that the merger "puts the credibility of the United Nations and the Security Council and independent countries in front of a real test in which they must choose between their bias for terrorism represented by al-Qaida ... or acknowledge the Syrian people and government's right to combat terrorism."
The U.S. designated Nusra Front a terrorist organization in December, and said then that the group had ties to al-Qaida. The Syrian group's public oath of allegiance to al-Qaida is unlikely to prompt a shift in international support for the broader Syrian opposition.
The U.S. and its allies are already working to try to counter the rising influence of Nusra Front and other Islamic extremists in the civil war by boosting their support for rebel factions deemed to be more moderate.
On Wednesday, U.S. Secretary of State John Kerry met with Syrian opposition leaders in London to discuss ways to step up aid to rebels.
So far, the U.S. and its allies have helped create the opposition umbrella group, the Syrian National Coalition, in the hope that it will serve as the united face of those trying to unseat Assad and administer much of the territory in northern Syria that rebels have managed to pry away from regime forces in the past year.
The U.S. and other countries also have stepped up covert support for rebels on the ground by helping to coordinate shipments of new weapons and training rebels in Jordan, officials say. Those receiving training are mainly secular Sunni Muslim tribesmen from central and southern Syria who once served in the army and police.
Nusra Front, which has welcomed militants from across the Muslim world into its ranks, has made little secret of its links across the Iraqi border. The group, which wants to oust Assad and replace his regime with an Islamic state, first emerged in a video posted online in January 2012. Since then, it has demonstrated its prowess — and ruthlessness — on the battlefield.
It has claimed responsibility for many of the deadliest suicide bombings against Syrian government institutions and military facilities. The group's success helped fuel a surge in its popularity among rebel fighters, although it has also emerged as a source of friction with more moderate and secular brigades in Syria.