Darfur's most-wanted rebel leader vowed Monday to keep up his offensive, saying he can exhaust Sudan's military by fighting it across Africa's largest nation. In a telephone interview with The Associated Press, Khalil Ibrahim explained his military success so far by saying, "We are more spread out and we move fast." The speed of his forces was widely credited with allowing Ibrahim's men to reach the outskirts of Khartoum to launch an attack Saturday. They set out from the Darfur and Kordofan regions under cover of night in pickup trucks and, according to Ibrahim, vehicles similar to those used by the army. They were spotted by the military but outran pursuers as they raced across the vast arid terrain of central Sudan with little obstruction. Ibrahim spoke while saying he was on the run in the capital Khartoum's twin city of Omdurman, where his Justice and Equality Movement launched a daring raid on Saturday. It is the closest Darfur's rebels have ever gotten to the seat of the government. "I am still in Omdurman. I am not safe but I am with all my forces," said Ibrahim, countering government claims that the assault was quashed. He said reinforcements were on their way. Gunfire could still be heard in Khartoum Monday morning. The attack, which Khartoum said it repelled, has shocked the government, which is conducting a full scale manhunt for Ibrahim and cracking down on other opposition figures. Islamist opposition politician Hassan al-Turabi, accused of links to JEM, was detained and questioned Monday, before being released uncharged. JEM has emerged as the most effective rebel movement in Darfur, where ethnic Africans took up arms against the government in 2003 to protest discrimination. Ibrahim declined to explain how his forces managed to attack a city hundreds of kilometers from their bases in Darfur, but he hinted at allies inside Khartoum itself. JEM, unlike other rebel movements in Darfur, has also succeeded in expanding its operations out of Sudan's wartorn western region into the central province of Kordofan, next to the capital. Ibrahim's close family ties with the powerful Chad-based Zaghwa tribe has bolstered his ranks and military capabilities, especially as relations have declined between Sudan and its western neighbor. Sudanese President Omar al-Bashir has accused Chad of being behind the attack and reserved Sudan's right to retaliate, while his top adviser Nafie Ali Nafie said Ibrahim was just fooling himself. "Those who dream of taking over Omdurman are delusional and don't know the people of Sudan," he was quoted by the official news agency as saying Sunday. Sudanese television, also reported that Ibrahim had asked for Chad to send a helicopter to evacuate him. Chad, meanwhile, has closed its border with Sudan "to avoid any infiltration and suspicious movement," said Communication Minister Mahamat Hissene late Monday. The more than 1,000 kilometer (mile) long border between the two countries runs through some of the most inhospitable and remote countryside in the world and armed groups have long passed through it with impunity. Experts say Ibrahim's advance toward Khartoum was aimed at bringing the Darfur conflict home to the heart of the regime's powerbase and force it to confront the festering crisis, which has claimed 200,000 lives and displaced 2.5 million people. The question remains, however, whether Ibrahim can support his boast to harass Sudan's armed forces the length and breadth of this vast country. "I would be very surprised if this didn't put Khartoum in a continuous state of alert," said Eric Reeves, a Sudan expert and professor at Smith college. "But I don't believe this is a military strategy that really will take it to Khartoum in the long run, and I think the reprisals are going to be ferocious." JEM claims it has thousands of fighters, but most experts believe the number is in the hundreds, making pitched battles like the clash in Omdurman an expensive proposition. It does not, however, lack for military supplies, and observers have seen JEM fighters with heavy weapons such as rockets and anti-aircraft guns. AP reporters along the Chad-Sudan border have seen JEM fighters operating on both sides of the frontier. Reeves said Chad has in recent months bolstered JEM's military capability tremendously. Chadian President Idris Deby, a tribal relative of Khalil, has blamed Sudan for supporting a Chadian rebel march onto the presidential palace in February. On Monday, security forces briefly detained Islamic ideologue al-Turabi, head of the powerful opposition National Popular Congress, for alleged links to the Saturday assault. "They asked me about my relation with the ... Justice and Equality Movement," al-Turabi said to the al-Jazeera satellite news channel, after his release late Monday. "I told them I don't see any link between me and such investigation so I will not answer them." Al-Turabi, who has a doctorate from the Sorbonne, is one of the founders of religious politics in Sudan and used to head the Muslim Brotherhood. He and Ibrahim were once part of the regime, and as fellow Islamists, ideological allies. Ibrahim, however, broke with al-Turabi and the regime over the issue of ethnic rights for the people of Darfur and still have their differences. "My people in Khartoum ... are not related to Turabi," Ibrahim said. "I have people inside the army, security and police and students in the university." Observers say al-Turabi is also engaging in talks with the regime to re-engage in elections. Alex de Waal, a British expert on Sudan, said described Khalil's advances as a "bid for power." He said Khalil may want to keep up the pressure on the regime, but is unlikely to be able to withstand the response. "I think it was a miscalculation," he said. "the majority of Darfur rebels don't share that ambition ... they want peace for their places rather than wanting power in Khartoum for themselves." Ibrahim's plan is to keep the government on the defensive, but he doesn't say for how long. "The government can't keep up with the JEM," he said. "It will be exhausted ... We can move from the north, south, west and east freely."