A Yemeni court has sentenced 32 men, found guilty of planning to attack oil and gas installations in 2006, to between two and 15 years in jail.
Yemeni authorities say the militants have strong ties to al-Qaeda
Six defendants were tried in their absence, including three alleged al-Qaeda men who escaped from prison before the attempted attacks.
The prosecution said it planned to appeal to seek tougher sentences.
Yemen has seen frequent attacks in the past claimed by Islamists and has a reputation as a haven for militants.
The court in the capital, Sanaa, acquitted four of the 36 defendants, who had been charged with "forming an armed gang aimed at carrying out sabotage attacks".
They were also accused of involvement in the attacks on the Safer refinery in Marib and the al-Dhabba terminal in Hadramout.
But the prosecution was not satisfied and said it planned an appeal to seek harsher sentences.
"The penalties do not correspond to the crimes the convicts have committed," prosecutor Khaled al-Maouri told Reuters agency.
All 30 defendants had pleaded not guilty and three of them alleged in court they had been tortured and forced to sign confessions, according to the Yemeni official news agency Saba.
History of attacks
The attacks were foiled in September 2006 by Yemeni security forces, who blew up four car bombs before they reached their target.
Four suspected suicide attackers and one security guard were killed during the operation.
The attempted attacks came just a few days after al-Qaeda's number two Ayman al-Zawahiri called on Islamist militants to target oil facilities in the Gulf region.
Yemen, the ancestral homeland of al-Qaeda leader Osama Bin Laden, has been the scene of major attacks including strikes on the French oil tanker the Limburg in 2002, and a suicide attack on US warship Cole in 2000, which killed 17 US sailors.
Since the 11 September 2001 attacks on the US, authorities have cracked down on suspected al-Qaeda members, and the US has supplied equipment and training for Yemen's security forces.