An attack by "Al-Qaeda elements" at a gas export terminal in Yemen killed five soldiers on Sunday, a military source said, as Washington kept its embassy in Sanaa closed.
"The attackers arrived in a car at the army checkpoint near the Balhaf terminal. They opened fire with automatic weapons, killing five soldiers before fleeing," the source told AFP on condition of anonymity.
A sixth soldier has disappeared after the attack and is "considered missing in action," the source said.
The soldiers belonged to an army unit responsible for security at the Balhaf terminal in Yemen's Shabwa province, part of which is run by French company Total.
The terminal, through which the bulk of Yemen's gas exports pass, was not among the reported targets of a large-scale Al-Qaeda plot that Yemeni authorities say they foiled in recent days.
But a government spokesman said that pipelines leading to the terminal were among the targets.
Al-Qaeda also plotted to assault the Canadian-run Mina al-Dhaba oil terminal in Hadramawt province further east and take staff hostage, including Western expatriates, spokesman Rajeh Badi told AFP.
A nearby export facility for oil derivatives was also targeted, Badi said.
Sunday's attack follows a wave of US drone strikes against suspected Al-Qaeda militants in Yemen that has killed 38 people since July 28.
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The latest strike hit late on Saturday north of the port city of Aden, killing two people and wounding one.
The intensification of the US drone war in Yemen came as a security alert prompted Washington to close 19 embassies and consulates in the Middle East and Africa.
Communications intercepts reportedly included an attack order from Al-Qaeda leader Ayman al-Zawahiri to Yemen-based Al-Qaeda in the Arabian Peninsula.
AQAP is considered by Washington to be the deadliest branch of the global extremist network.
Both Washington and London pulled out diplomatic personnel from Sanaa on Tuesday, citing intelligence reports of an imminent AQAP attack.
The United States said on Friday that all of the embassies it shut would reopen this week, except the mission in Yemen.
The US embassy in the UAE capital, Abu Dhabi, and its consulate in Dubai reopened on Sunday, as did the embassy in Libya.
But other Gulf missions in Bahrain, Kuwait, Qatar and Saudi Arabia remained shut due to Muslim Eid holidays.
State Department spokeswoman Jen Psaki said Washington would also keep its consulate in the Pakistani city of Lahore closed, after pulling out staff on Thursday.