American fighter jets thundering over Sarnia are raising questions about safety and Canadian sovereignty, according to the city's mayor.

Mike Bradley has written a letter to Canada's Foreign Affairs Minister Chrystia Freeland about two A-10 Thunderbolt II planes, nicknamed "Warthogs" that have been roaring along the Canadian side of the border before flying through downtown Sarnia and over the chemical valley since the summer.

"It's the principle of our sovereignty," he said. "It's also very frightening. I've heard a couple of stories where it's petrified people."

Mike Bradley

Sarnia Mayor Mike Bradley said American jets flying over his city are a sign of increased militarization along the border. (Facebook)

Bradley said this isn't the first time the planes, which take off from nearby Selfridge Air National Guard Base, have been spotted over Sarnia. He complained to the federal government about the same problem seven years ago and received an apology from the base commander and a promise it wouldn't happen again.

"Well it's happened again and it's been happening all summer," he said, adding he's hopeful Freeland will be able to explain why the jets have returned or to encourage the Americans to stick to their side.

Border becoming more 'unfriendly'

The mayor said while not everyone is disturbed by the flyovers, he sees them as a problem and sign of changes along the boundary between the two countries.

"I'm personally disturbed we've militarized the border between Canada and the U.S. way beyond what I think it needs to be," he said. "We've made it a much more unfriendly border."

Global Affairs Canada says jets allowed

While Bradley considers U.S. military aircraft flying over his city an affront to Canadian sovereignty, the federal government says it is not illegal.

"Under legislation that came into force on June 1, 2009, military aircraft of the United States may fly over Canada. This is a reciprocal agreement and reflects our close defense relationship," Global Affairs Canada spokesperson John Babcock said in a statement to CBC News.

"Canada will continue to work with our U.S. counterparts to address questions about the U.S. flight paths."

CBC has also contacted the Selfridge Air National Guard Base but has yet to receive a reply.