The violence, which underscores the almost total lack of security in the North African state, followed an overnight rocket attack on the government's offices in the capital, Tripoli.
In Geneva, the International Committee of the Red Cross confirmed that one its workers, Michael Greub, 42, was killed by gunmen at around noon (10am GMT) in the coastal city of Sirte.
Greub, who headed the agency's office in Misrata, 200 kilometres from Sirte, was leaving a meeting with two colleagues when the attackers shot at their vehicle at "point-blank" range, according to ICRC spokesman Wolde Saugeron.
The other two staff members were unharmed.
"The ICRC vigorously condemns this heinous attack," said Director-General Yves Daccord, deploring the loss of "a devoted humanitarian who spent many years of his life helping others."
We condemn in the strongest possible terms the murder of Michael Greub: http://t.co/fcjFG81LqC Our thoughts are with his loved ones.
— ICRC (@ICRC) June 4, 2014
Earlier, former general Khalifa Haftar, whose repeated deadly assaults on jihadists in Benghazi have earned him threats of reprisal, was lightly wounded in a suicide bombing.
Three of his loyalists were killed in the attack on a villa outside the eastern city, one of his commanders said.
It was the first attack on Haftar since he launched "Operation Dignity," aimed at eradicating "terrorists" in Benghazi on May 16th.
Libya's second city was the cradle of the revolt that toppled dictator Moammar Kadhafi in 2011 and has become a bastion of jihadists accused by Washington of involvement in a deadly assault on the US consulate there in 2012.
"A suicide bomber in a vehicle packed with explosives attacked a villa where we had gathered," said General Sagr al-Jerushi, who heads the air wing Haftar has used in his attacks on the jihadists.
"Three of our soldiers were killed," Jerushi said, adding that he had also been "lightly wounded."
Haftar later appeared on Libyan television, his left hand bandaged, and vowed "reprisals."
"We are ready to continue (our operation) until we achieve our objectives," he said.
Following the attack, witnesses reported air strikes on targets in Benghazi.
Haftar spokesman Mohamed Hejazi blamed "terrorists and extremists" for the suicide bombing, without naming any particular group.
Ansar al-Sharia, a radical Islamist group blacklisted by Washington as a "terrorist organization" for its alleged role in the consulate attack, has borne the brunt of Haftar's offensive and had threatened to retaliate.
Al-Qaeda too has threatened the former general, who spent years in US exile before returning to join the anti-Kadhafi rebellion.
Its North Africa affiliate — Al-Qaeda in the Islamic Maghreb — labelled him an "enemy of Islam" on Sunday and urged Libyans to fight him.
Haftar claims his forces represent the legitimate Libyan army and has won the support of regular army units inside Benghazi, including the special forces, for his offensive.
Their latest assault, on Monday, left at least 21 people dead and 112 wounded.
Even though Haftar denies any political ambitions, the outgoing Islamist-led parliament accuses him of plotting a coup and has branded him an outlaw.
Since 2011, Libya has been awash with heavy weapons and former rebel militias control swathes of territory across the country.
As a result, the central government has struggled to impose its authority.
Haftar's campaign against the Islamists has struck a chord among many Libyans angry at the militias, and there have been repeated street demonstrations in his support in Benghazi and Tripoli.
But he remains a figure of suspicion for many veterans of the uprising, with his US exile leading some of them to accuse him of links to the CIA, something also claimed by the Kadhafi regime.
Washington has denied any role in Haftar's offensive in Benghazi.
In Tripoli, MP Abu Bakr Maddur was kidnapped as he was leaving the justice ministry, according to a parliamentary source.
And a rocket overnight hit the government's head offices, where new Prime Minister Ahmed Miitig was to hold a cabinet meeting, causing damage but no casualties, an official said.
Miitig was not in his office at the time.
Despite the attack, Miitig travelled to his offices on Wednesday to hold a meeting with his ministers, an AFP journalist said.
Devastated. Outraged. Our friend & colleague, Michael Greub, was a devoted #humanitarian who spent many years of his life helping others.
— Yves Daccord (@YDaccordICRC) June 4, 2014