Yemen's President Abd-Rabbu Mansour Hadi has condemned the "assassination" of the head of a Yemeni liberal political party in the capital Sanaa.
Mohammed Abdel Malek al-Mutawakel, secretary-general of the Union of Popular Forces party, was "fatally shot in the neck" by a gunman on a motorcycle on a street in central Sanaa, his daughter Radhia told the AFP news agency.
A medical source confirmed the death. It was not immediately clear why Mutawakel was targeted.
Mansour said on Monday that authorities would "pursue the perpetrators and hold them accountable", adding that the shooting aimed to "create chaos in Yemen".
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The attack came a day after Shia rebels and their political rivals, including Mutawakel's party, signed an agreement to form a new technocratic government to resolve the country's political crisis.
The Houthi rebels also denounced the killing of Mutawakel, blaming authorities for failing to provide enough security for the political leader.
Al Jazeera's Omar Al Saleh, reporting from Sanaa, said the killing will further inflame tensions in the conflict-torn country.
"This assassination is considered to be a blow for the breakthrough that happened regarding forming a government. [Mutawakel] was seen as a link between the warring parties," our correspondent said.
One of the founders of the current ruling party in Yemen before he turned to opposition, Mutawakel had been helping to negotiate a peace deal between Houthi rebels and the government.
But Mutawakel was also a controversial figure, accused of backing the Houthis behind the scenes.
Meanwhile, further west in the Red Sea port city of Hudeida, suspected al-Qaeda fighters killed 18 Yemeni troops and captured 15 others, security and tribal sources said on Sunday.
The overnight attack targeted security headquarters in Hudeida, which Shia Houthi rebels seized in mid-October as they spread across several towns and cities, facing almost no resistance from government forces.
Al-Qaeda frequently carries out attacks on Yemen's security forces and it has been locked in deadly battles with the Houthi rebels.
Yemen, a key US ally, has allowed Washington to conduct drone strikes against al-Qaeda on its territory.
Source: Al Jazeera and agencies