SEOUL, June 12 (Reuters) - North Korean leader Kim Jong Un has said his country is an "invincible comrade-in-arms" with Russia in a message to President Vladimir Putin, state media KCNA said on Wednesday, amid speculation over Putin's impending visit to North Korea.

Marking Russia's National Day, Kim said his meeting with Putin at a Russian space launch facility last year elevated the ties of their "century-old strategic relationship".

The message came after Russia's Vedomosti newspaper on Monday reported Putin would visit North Korea and Vietnam in the coming weeks.

An official in Vietnam told Reuters the Vietnam trip was planned for June 19 and 20, but has not yet been confirmed. The Kremlin has said Russia wants to foster cooperation with North Korea "in all areas" but has not confirmed the date of the visit.

Kim travelled to Russia's Far East last September, touring the Vostochny Cosmodrome space launch centre, where Putin promised to help him build satellites.

Kim also lauded Russia for achieving results on its efforts to build a strong country despite by "suppressing and crushing all the challenges and sanctions and pressures of hostile forces".

Pyongyang and Moscow have increasingly stepped up diplomatic and security relations, hosting government, parliamentary and other delegations in recent months.

A group of North Korean officials in charge of public security was set to visit Russia this week.

Officials in Washington and Seoul have accused North Korea of shipping weapons to Russia to support its war against Ukraine in exchange for technological aid with its own nuclear and missile programs.