Berlin has so far held back from sending them or allowing other NATO countries to do so.

After denying the blast just inside Poland's border could have been caused by his country, Ukraine's leader focuses on his demand to join the investigation.

North Korea fired a ballistic missile on Thursday as it warned of "fiercer military responses" to U.S. efforts to boost its security presence in the region with its allies, saying Washington is taking a "gamble it will regret".