US Coast Guard planes and vessels scrambled a rescue mission today after yesterday's mid-air plane crash in Alaska, as the toll was revised.

The coastguard sent an MH-60 Jayhawk helicopter crew and two 14m Response Boat-Medium crews from its base in Ketchikan.

Two sightseeing planes collided, leaving at least four people dead and two missing. An Australian and a Canadian are the two cruise ship passengers missing, Princess Cruises said.

All 14 passengers on the two planes came from the cruise ship Royal Princess, which was on a seven-day trip from Vancouver, British Columbia, to Anchorage. Initially, the company said five people had died.

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The 10 rescued passengers, all Americans, were being treated at PeaceHealth Ketchikan Medical Centre, USA Today reported, and one of those patients was in critical condition.

Coast Guardsmen were aided by volunteers in the search, said Captain Stephen White, the commander of the Juneau sector.

"In a remote area such as this, given our limited resources, we rely on our partner agencies and appreciate the support that good Samaritans have rendered to this point," White said.

The two planes collided yesterday at 1.08pm local time, according to Princess Cruises, about eight nautical miles off Ketchikan, Alaska.

The names of passengers killed have not been released. Canadian officials said one of the country's citizens was among the dead.

One plane, a de Havilland Otter seaplane operated by Taquan Air, was carrying 10 guests from the cruise ship as well as a pilot, returning from a tour of the nearby Misty Fjords National Monument. The other aircraft, a de Havilland DHC-2 Beaver operated by an "independent tour," according to the cruise line, carried four Royal Princess passengers and a pilot.

The Beaver appeared to have crashed on a steep rocky shoreline, partially submerged upside down in seawater, volunteer rescuer Chris John told the Anchorage Daily News.

The Coast Guard said it is "unaware" of why and how the planes collided. The National Transportation Safety Board has sent a team to Alaska to investigate the crash.

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In a statement, Taquan Air said it was "devastated" by the incident and suspended all scheduled flights as it cooperates with investigators, the Daily News reported.

The collision was the second crash involving Taquan in the area in the past year. In July, after a plane crashed into a mountainside, investigators concluded a pilot turned off a warning system that alerts to such collisions, the Daily News reported. All 11 people onboard survived, though some of them suffered serious injuries.

A 2015 crash in the same area was eerily similar to yesterday's incident.

A plane with cruise line passengers crashed into a mountain while returning from the Misty Fjords National Monument, killing all eight passengers and the pilot. Lax standards and flying despite poor weather led to the crash, investigators concluded, according to the Daily News. The plane was operated by Promech Air. It was bought by Taquan the next year.

- additional reporting news.com.au