Even as Libyan dictator Moammar Gadhafi's grip on the country was slipping fast with rebels making headway into the centre of Tripoli and international pressure on the four-decade ruler to step down reached its zenith, there are reports that Gadhafi's son Seif Al-Islam has been captured by the rebels while another son, Mohammed, has surrendered.

Al Jazeera also reported that Gadhafi’s whereabouts are unknown, though a rebel leader said the Libyan strongman was unharmed.

While the National Transitional Council told Al-Jazeera television from Benghazi that he had information that Seif al-Islam has been captured, the channel confirmed that Gadhafi's son Mohammed had surrendered to the rebels.

The chairman of the Transitional Council said Seif was kept in a secure place under close guard and that he will be handed over to the judiciary.

Egypt's Muslim Brotherhood's Web site said Gadhafi regime's green flags are being torn down and opposition flags waved in the center of Tripoli.

An International Criminal Court (ICC) Prosecutor was quoted by AFP as saying that Seif al-Gadhafi was in detention. I have received confidential information stating he has been arrested, ICC prosecutor Luis Moreno-Ocampo said.

Seif, once the would-be successor to Gadhafi, had an ICC warrant on him for alleged crimes against humanity. His role in his father's government was significant, though Mohammad had less responsibility, serving as the chairman of Libya's main state-run telecommunications firm.

Gadhafi, whose whereabouts remain unknown, made an impassioned appeal to the tribal leaders to March to Tripoli and defend the regime in a brief audio message on Sunday night.

Meanwhile, the United States and other global powers stepped up pressure on Gadhafi to give up the fight. President Barack Obama said the regime should recognize that their rule has come to an end. Tonight, the momentum against the Gadhafi regime has reached a tipping point. Tripoli is slipping from the grasp of a tyrant, Obama said.

Gadhafi needs to acknowledge the reality that he no longer controls Libya. He needs to relinquish power once and for all, the President said.

British Prime Minister David Cameron's Web site had a statement that read: It is clear from the scenes we are witnessing in Tripoli that the end is near for Gadhafi. He has committed appalling crimes against the People of Libya and he must go now to avoid any further suffering for his own people.

Though two of the most influential Gadhafi sons are in rebel custody some key operatives of his regime are still at large. The most important among them is Abdullah al-Senussi, who has been Gadhafi's intelligence chief for a long time. He is also Gadhafi's brother-in-law. Al Jazeera reported that, according to many Arabic newspapers, Senussi might have fled Tripoli. Senussi had been accused by the ICC of carrying out a campaign of murder, mass arrest and torture, the report said.