* ICRC official seized on way to south Yemen port
* Four Islamist militants killed in air strike
SANAA/ADEN, April 22 (Reuters) - Armed men kidnapped a French an aid worker of the International Committee of the Red Cross as he was travelling from north Yemen to the Red Sea port city of Hudaida, the ICRC said on Sunday.
It was not immediately clear who was responsible for the kidnapping but seizing foreigners or Yemenis in order to secure releases of jailed relatives is common in the impoverished Arabian Peninsula state. Most hostages are freed unharmed.
Dibeh Fakhr, an ICRC spokeswoman in Sanaa, said the Frenchman, who works in the northern city of Saada, was kidnapped late on Saturday some 30 km (18 miles) from Hudaida.
"He was with two Yemeni drivers who the kidnappers released shortly afterwards," Fakhr said. "Until now we have no contact with the kidnappers or our employee."
Yemen slid into disorder after the outbreak of protests more than a year ago that eventually ousted autocratic President Ali Abdullah Saleh under a deal brokered by Yemen's rich Gulf neighbours, including regional heavyweight Saudi Arabia.
But the U.S. ambassador to Sanaa, Gerald Feierstein, said the country's new president, Abdu-Rabbu Hadi Mansour, was making headway in fighting al Qaeda, which controls swathes of land in the south, including Zinjibar, capital of Abyan province.
"We have seen the Yemeni military becoming more aggressive (against Qaeda)," he told reporters on Sunday.
Feierstein renewed a warning he made in February about alleged Iranian involvement in Yemen. "They are supporting elements within Yemen both in the north and south who are using violence to undermine the stability of the country," he said.
Iran denies supporting militants in Yemen.
In Lawdar, 18 Islamist militants were killed in three separate air strikes, residents and officials said, as battles with government forces continued around the southern city for the third week.
The defence ministry said in a text message at least three of the dead were Somalis. A local official said six fighters were killed in an air strike on a mosque outside Lawdar.
On Saturday, a soldier was killed and 10 wounded in clashes with Ansar al-Sharia, the Qaeda-linked group, a security official said, adding that one militant was also killed.
Fighting continued in Zinjibar, where the army killed at least 18 militants on Friday. A defence ministry website said government forces advanced in eastern and southern parts of the city that the militants seized a year ago.
And in the eastern province of Maarib, two suspected Qaeda militants were killed when military planes bombed two vehicles, a local official told Reuters.
(Reporting by Tom Finn and Mohammed Ghobari in Sanaa and Mohammed Mukhashaf in Aden; Writing by Mahmoud Habboush and Sami Aboudi; Editing by Michael Roddy)
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