WASHINGTON - Signs that Canadian citizens were involved in the attack by hostage-taking Islamist militants on a remote gas plant in Algeria are of great concern to American authorities, US intelligence officials said on Thursday.While Algerian authorities apparently have not yet provided Western governments with cast-iron proof, a senior US intelligence official said, "We're taking very seriously the reports of the two Canadians' involvement."Washington has also received intelligence indicating that Mokhtar Belmokhtar, the one-eyed militant who allegedly organized the gas plant attack, and other militants belonging to, or affiliated with al-Qaida's North African branch are plotting new attacks on Western interests, both in the region and beyond, US officials said.But the information indicates, for now, that such plotting is "aspirational" - in discussion or planning phases - rather than operational. The intelligence officials, who spoke on condition of anonymity, said that meant they were not aware of plots actually in progress to attack US or other Western interests.