Image: Javier Cebollada / EPA

NATO has offered an explanation as to why two Spanish F-18 Hornet fighter jets breached Finnish airspace on Tuesday.

The military alliance says that the F-18's were scrambled to intercept and identify two Russian planes that were flying close to Estonian airspace. During the interception flight, the NATO aircraft briefly entered Finnish airspace, NATO says.

The Spanish planes took off from Estonia's Ämari Air Base. They identified the Russian planes as a MiG-31 fighter jet and an Antonov AN-26 transport plane.

Finnish planes asked to take over

NATO says that Finnish jets also flew over to identify the Russian aircraft, and were then asked to take over in tracking the Russian planes.

Estonia and Spain are full NATO members, while Finland has an "enhanced" membership in the alliance's Partnership for Peace.

The alliance said the fighter planes violated the airspace when they were in the process of asking Finland, which is not a NATO member, to take over the chase.

"In handing over the intercept to the Finnish jets, the Spanish jets accidentally entered Finnish airspace. NATO's Air Command has explained the incident to the Finnish Air Operations Centre to improve future coordination," said NATO spokesman Dylan P. White.

Kirsti Haimila, a Senior Adviser at the Finnish Ministry of Defence, says the NATO planes were in Finland's airspace for about a minute.

Most incursions by Russian planes

According to the ministry, 35 violations of Finnish airspace were detected and announced between 2005 and 2016. Before 2004, such incidents were not reported.

Most incursions have only lasted a few minutes.

Russian aircraft accounted for the largest number of violations, followed by those from Sweden, the US and Norway.

In 2016 there were four reported incidents, two involving Russian planes, with the others Danish and Swedish. There was another Swedish incursion this past January. Russian incursions were detected in 2013-15.

The ministry says that a Finnish plane has only breached another country's airspace once in this decade. That occurred in May 2010, when a Finnish F-18 crossed into Swedish airspace.