(Adds suicide bomb, clashes, militant denial)
ADEN, Yemen, April 15 (Reuters) - A roadside bomb targeting a Yemeni security patrol killed three children on their way to school in a southeastern province on Sunday, the Defence Ministry said, blaming the deaths on al Qaeda insurgents.
Qaeda-linked militant group Ansar al-Sharia denied involvement in the attack in an emailed statement, adding they were being framed to undermine their popularity.
Separately, a tribal source said a suicide bomber drove up to a checkpoint manned by tribesmen and blew himself up, killing two tribesmen near the southern town of Lawdar, where the army launched an offensive against Islamist militants last week.
The militant group separately claimed responsibility for this attack, which took place in al-Hodn.
Six militants and two tribal fighters were also killed in further clashes near Lawdar, in an area called al-Minyasa, tribal sources said.
Still reeling from a year of political upheaval that unseated former president Ali Abdullah Saleh, Yemen is grappling with militants who have exploited weakened central government control to gain a foothold in the country's south.
The Interior Ministry said on its website security forces were on alert for a potential plot by al Qaeda targeting "vital and government installations" in the southern province of Dalea.
It gave no further details but called on residents to report any "terrorist" activities.
Referring to the roadside bomb that killed three children, the Defence Ministry said on its website the explosives had been placed on a road in al-Qatn city in Hadramout.
The United States and Saudi Arabia fear protracted political upheaval in Yemen could give militants room to plot attacks within the region and beyond.
They threw their weight behind a power transfer plan under which Saleh handed over power to his then-deputy, Abd-Rabbu Mansour Hadi who took office this year. (Reporting by Mohammed Mukhashaf; Writing by Rania El Gamal and Isabel Coles; Editing by Maria Golovnina)