Soon after he spoke, four powerful explosions rocked the centre of Tripoli, the target of more and more intensive air raids by NATO warplanes for more than a week, an AFP journalist reported.
The first two blasts rumbled across the city at around 9:45 pm (1945 GMT) as aircraft were heard in the night skies overhead, followed by another two 10 minutes later.
It was not possible to determine the targets, the reporter said.
Ibrahim ruled out the embattled strongman stepping down from power.
"If Kadhafi goes, the security valve will disappear," he said.
"Kadhafi's departure would be the worst case scenario for Libya," he told reporters, and warned of "civil war."
Ibrahim also denied that South African President Jacob Zuma, who met Kadhafi in Tripoli on Monday, had discussed an "exit strategy" with him.
Zuma "never discussed any exit strategies as they have been described in the media," the spokesman said.
Earlier, a statement from the South African presidency in Pretoria said Kadhafi would not leave Libya despite growing international pressure and intensified NATO strikes on his regime.
"Colonel Kadhafi called for an end to the bombings to enable a Libyan dialogue. He emphasised that he was not prepared to leave his country, despite the difficulties," Zuma's office said in a statement.
The South African president said raids by NATO, which is enforcing a no-fly zone over Libya and protecting civilians from a government crackdown under a UN mandate, were undermining African mediation efforts.
South African Foreign Minister Maite Nkoana-Mashabane called for an immediate ceasefire after Zuma said Kadhafi was "ready" to implement an African Union peace plan already rejected by NATO and the rebels.
In the rebel capital Benghazi, in eastern Libya, Italy's foreign minister said Tuesday Kadhafi's regime was already staring at defeat.
"The Kadhafi regime is finished, he must leave office, he must leave the country," Franco Frattini told a joint news conference with Ali al-Essawi, the rebels' foreign affairs chief.
"His aides have left, he has no international support, the G8 leaders reject him, he must go."
Frattini was speaking ahead of a ceremony to inaugurate a new Italian consulate in the eastern city, in another major blow to Kadhafi after NATO insisted his "reign of terror" is nearing an end.
Italy, the former colonial ruler of Libya and strategic economic partner with Kadhafi's regime, has joined international calls led by Britain, France and the United States for the Libyan leader to go.
In Washington, State Department spokesman said US Secretary of State Hillary Clinton will visit the United Arab Emirates on June 9 for a meeting of the Libya contact group.
"This meeting will build on the last contact group meeting held in Rome," and will allow the United States and its partners to discuss implementation of UN Security Council resolutions 1970 and 1973, Mark Toner told reporters.
UN under secretary general B. Lynn Pascoe told the UN Security Council meanwhile that at least 1,200 people have been killed or are missing after trying to flee Libya by boat since the start of the uprising against Kadhafi mid-February.
At the same time, the official TAP news agency reported from Tunis Tuesday that five more officers have joined the flow of defectors from Kadhafi's regime, arriving at the weekend in neighbouring Tunisia.
The colonel and four lieutenant colonels as well as four rank-and-file soldiers arrived by boat on Sunday, the report said.
In Rome on Monday, five generals, two colonels and a major announced they had defected from Kadhafi's forces, calling on other officers to follow their example.
Abdel Rahman Shalgham, a former foreign minister who was Tripoli's UN representative before switching sides, told a news conference that around 120 officers had defected in recent days.