By Roger Hardy BBC Middle East Analyst

The explosion this week on a French supertanker off the coast of Yemen - and the subsequent killing of a US marine in Kuwait - are leading experts to question whether al-Qaeda is showing signs of resurgence.

There is still no proof that Sunday's explosion was an act of terrorism, but the early assessment of a US investigator sent to the scene is that it was.

The incident has inevitably evoked memories of the attack on a US warship, the Cole - also in Yemeni waters - two years ago.

Experts are in no doubt that attack was the work of al-Qaeda - and believe the group may now regard European, as well as American, targets as legitimate.

Future attacks

On Tuesday the al-Jazeera TV station broadcast a statement allegedly made by Ayman al-Zawahiri - al-Qaeda's number two in command after Osama Bin Laden.

A statement from Ayman al-Zawahiri warned of further attacks

The statement threatened future attacks against America and its allies, and claimed credit for recent attacks against French and German targets - a claim which seems to refer to an attack in Tunisia which killed German tourists, and one in Pakistan which killed French engineers.

US officials say they regard the recording as authentic, and think it may be of more recent origin than a tape said to be of Osama Bin Laden, broadcast by al-Jazeera the previous day.

The Americans have had some recent successes - including the capture last month in Pakistan of a senior al-Qaeda figure, Ramzi bin al-Sheeba.

On Wednesday a court in Chicago indicted the head of a big Islamic charity, Benevolence International, accused of funnelling tens of thousands of dollars to al-Qaeda.

Meanwhile arrests of al-Qaeda suspects continue in a number of European, Arab and Asian capitals.

But what worries Western investigators is that al-Qaeda still seems to have part of its leadership intact, and still seems capable of carrying out operations.

And many experts are warning that if America goes ahead with a military assault against Iraq, support for the group and its agenda is likely to grow.