(Reuters) - Romania's foreign ministry summoned the Russian charge d'affaires after the discovery on Romanian soil of new fragments of a drone similar to those used by the Russian military, the Agerpres website reported on Sunday.

On Saturday, Romanian authorities found the second set of drone fragments to have crashed in the NATO member state in a week, amid Russian attacks on Ukraine's river ports, just hundreds of metres from the Romanian border.

The attacks have increased security risks for NATO whose members have a mutual defence commitment.

"...The charge d'affaires of the Embassy of the Russian Federation in Bucharest was urgently summoned on Saturday to the Ministry of Foreign Affairs, at the level of state secretary," the foreign ministry said in a statement sent to Agerpres.

"State Secretary for Strategic Affairs Iulian Fota conveyed the protest of the Romanian side over the violation of Romania's air space following the identification on (our) territory, near the border with Ukraine, of some drone fragments similar to those used by Russian forces in the aggression against Ukraine."

On Saturday, President Klaus Iohannis said the discovery of the fragments pointed to an unacceptable breach of Romania's air space.

NATO Secretary General Jens Stoltenberg said Russian strikes near the border were "destabilising" even if there was no indication Russia intended to hit Romania, a NATO member state.

Since July, when Moscow abandoned a deal that lifted a de facto Russian blockade of Ukraine's Black Sea ports, it has repeatedly struck Ukrainian ports that lie across the Danube River border from Romania.

Ukraine is one of the world's biggest grain exporters and the Romanian Black Sea port of Constanta is now Kyiv's largest alternative export route, with grains arriving by road, rail or barge across the Danube.

(Reporting by Alan Charlish in Warsaw; editing by Elaine Hardcastle and Mark Heinrich)