BEIRUT — Syria’s prime minister has fled the country, activists and Syria’s official media reported Monday, in what appeared to be the highest-level defection from President Bashar Assad’s government thus far.

Jubilant opposition figures said Riad Farid Hijab had defected to neighboring Jordan along with at least two ministers and three military officers — 10 families in all, opposition leaders said.

Al-Jazeera television carried what it said was a statement from a spokesman for Hijab saying he had ‘‘joined the ranks of the freedom and dignity revolution.’’

Mohammed Otari, Hijab’s spokesman, said the prime minister began planning his break from the regime two months ago, when Assad offered him the post and an ultimatum: Take the job or die, the Associated Press reported.

Hijab, long a loyalist to Assad’s Ba’ath party, had become a supporter of the opposition, Otari said from Amman. He said the prime minister and his family planned to travel from Jordan to Qatar.

In Washington, White House spokesman Jay Carney said the defections were ‘‘a sign that Assad’s grip on power is loosening.’’

‘’That the titular head of the Syrian government has rejected the ongoing slaughter being carried out at Assad’s direction only reinforces that the Assad regime is crumbling from within and that the Syrian people believe that Assad’s days are numbered,’’ he said.

Syria’s state-run news media tried to preempt the announcement by saying that Hijab had been fired as prime minister, a post he had held for less than two months. The Syrian government immediately announced a replacement: Omar Ghalawanji, a longtime official with an engineering degree who had been deputy prime minister.

State media also disputed news reports that the finance minister had joined the opposition, with the SANA news agency quoting the minister Monday as calling such reports untrue. Opposition activists did not immediately identify the other officials said to have defected.

Hijab appeared to be the highest-ranking civilian official to defect since the conflict started 17 months ago.

The Syrian announcement of his dismissal came hours after a bomb explosion was reported at the main state television building in Damascus, the capital.

Omran al-Zoubi, Syria’s information minister, said the bombing at the television station, which did not knock the station off air, was insignificant. “Nothing can silence the voice of Syria or the voice of the Syrian people,’’ said Zoubi.

Still, the explosion, near a busy traffic circle in central Damascus, offered another sign of the rebels’ ability to breach state institutions. On July 18, a bomb at the state security headquarters in Damascus killed four of Syria’s top military and security officials.

In Aleppo, rebels said, Syrian jets were dropping bombs on the city Monday. Fierce fighting also was reported in other parts of the country.

Iran’s role in the conflict is also growing more tangled. Syrian rebels who captured 48 Iranians near Damascus over the weekend said three had been killed in government shelling and the rebels threatened to kill the rest if the shelling did not stop. Earlier the Iranian government called for a broad, international emergency meeting on Syria in Tehran on Thursday.

The rebels holding the captives — from the Bara’a Brigade, one of the myriad groups fighting under the banner of the Free Syrian Army — say the Iranians are members of Iran’s elite Revolutionary Guard. Iran says they are pilgrims who had been visiting an important Shi’ite shrine near the neighborhood of Tadamon, where fighting has raged for weeks.

Iran’s call for an international meeting came in the diplomatic vacuum created by the international deadlock over the crisis and the failure of peace efforts led by Kofi Annan, the United Nations and Arab League special envoy who last week said he would not renew his mandate at the end of August.

Iran’s deputy foreign minister, Hossein Amir Abdollahian, said that 10 countries that all have a ‘‘realistic position’’ on Syria would be participating in an emergency meeting at the ministerial level, scheduled for Thursday.

“The Islamic Republic of Iran believes that a total halt to the violence, and national dialogue, are the solution to control the crisis in Syria, and to that end Iran is organizing this meeting,’’ the state Islamic Republic News Agency quoted Abdollahian as saying.

Iran, a staunch ally of Assad, has not been invited to international meetings on Syria. Iran’s foreign minister, Ali Akbar Salehi, has promoted the idea of presidential elections for 2014 and accused ‘‘regional countries’’ of supporting the Syrian rebels.

The departure of Hijab, a Sunni, cost the Assad government yet another piece of its claim to broad legitimacy. To reinforce loyalty, the Assad family has long stocked the Syrian government with members of its Alawite minority, but to placate other, larger groups, routinely placed non-Alawites in positions of limited authority. Hijab’s home area is the eastern town of Deir al-Zour, the scene of some intense fighting.

An activist who fled Syria and who said he had dealt frequently with Hijab during the initial protests last year, when Hijab was serving as governor of the coastal province of Latakia, said he seemed to have at least some sympathy early on with the opposition.

The activist, who identified himself only as Rami because he feared reprisals, said Hijab agreed to keep the military and police away from the first protests, and later, after arrests were made at subsequent demonstrations, Hijab helped get 15 people released.

Hijab’s defection reinforced rebel suggestions that Assad’s government was under severe strain.

‘‘This is someone who was very, very close, and they couldn’t keep him,’’ said Paul Salem, the director of the Carnegie Middle East Center in Beirut. ‘‘It doesn’t necessarily affect the basic security apparatus and the army, which is still holding the country together. The impact is not cataclysmic but it’s a sign of advanced decrepitude.’’