The Islamic Front said the attack was a prelude to a "large-scale operation" to secure territorial gains. The group is a coalition of insurgents including former Free Syrian Army fighters and members of harder-core Islamist factions, and it has clashed in many areas with al-Qaida-influenced insurgents.
The state news agency SANA said the "enormous attack" had rocked the Old City of Aleppo, destroying historical sites. Attackers blew up "tunnels they dug under archaeological buildings," SANA said.
State television identified the hotel as the Carlton. Images on the Internet depicted the hotel as a traditional building in pale stone with palm trees outside.
The hotel was built as a hospital during the Ottoman period that ended with World War I. It was later renovated and reopened as a hotel facing the historic citadel in Aleppo, Syria's largest city. Government forces had been billeted there for two years.
The Syrian Observatory for Human Rights, which is based in Britain and collates information from contacts inside Syria, said that Islamist forces had tunneled from areas held by rebels seeking the overthrow of President Bashar Assad. The hotel was "completely destroyed" and government soldiers suffered unspecified casualties, according to the observatory.
An activist group, the Shaam News Network, said government forces had been based in the hotel.
Aleppo has been carved into a checkerboard of areas held by rebels and government troops as the civil war has intensified. Government forces have launched a campaign of aerial bombardment on rebels, who have fired mortars and detonated car bombs.
The attack in Aleppo came a day after rebels in the city of Homs began an evacuation of positions taken as the revolt against Assad took root in March 2011. The evacuation was seen as a bitter defeat and emotional blow for antigovernment forces there, but it was not clear whether the bomb in Aleppo was intended as a direct response.
Homs is Syria's third-largest city and was one of the first in which protesters switched from street demonstrations to armed combat with government forces. Some of its neighborhoods were also among the first to be targeted by indiscriminate bombardment by government forces, a bellwether for the nation's descent into turmoil.
For the government, the rebels' evacuation was held up as proof that it could retake a major urban area through brute force and local talks.
Talal Barazi, the provincial governor in Homs, had told state media that 80 percent of the rebel fighters had left positions under the terms of the evacuation. The remaining fighters were scheduled to leave Thursday, when the center of Homs would be "declared a secure city" and reconstruction would commence, Reuters reported.
The attack on the hotel in Aleppo was the second of its kind in a little more than a month. In the earlier blast, explosives in a tunnel underneath the hotel damaged much of the building but did not force government troops to abandon it.
Tamam, an activist from Aleppo now living in Turkey, who did not wish to give his family name for fear of reprisals, said government forces had turned the hotel into a military base overlooking the old quarter of the city, using it as a base for mortar and sniper attacks.
Government supporters in Aleppo said in postings on Facebook that there may have been two tunnels packed with explosives, one under the hotel and the other in a separate neighborhood.
In video footage posted on YouTube, the Islamic Front showed what it said was an example of a similar attack this week in a tunnel underneath a government outpost in the city of Idlib, southwest of Aleppo. The footage showed a huge blast in which the Islamic Front said 35 government soldiers had died.