NATO chief Jens Stoltenberg on Wednesday thanked Turkey, Finland and Sweden for accepting his invitation "to engage in negotiations to find a united way forward."

"This is a good agreement for Turkey, Finland and Sweden, and it is a good agreement for NATO," he said.

His comments came after NATO invited Sweden and Finland to become members of the military alliance, according to a NATO communique published Wednesday.

"The accession of Finland and Sweden will make them (the allies) safer, NATO stronger and the Euro-Atlantic area more secure," the communique said, adding that the alliance also agreed a new strategic concept.

The communique described Russia as the "most significant and direct threat to the allies' security," a reaction to the massive deterioration in ties with Russia since its invasion of Ukraine.

The alliance pledged further help to Kyiv and agreed a package of support aimed at modernizing the country's defense sector.

At the same time, NATO decided to significantly strengthen its own deterrence and defense.

"Allies have committed to deploy additional robust in-place combat-ready forces on our eastern flank, to be scaled up from the existing battlegroups to brigade-size units, where and when required underpinned by credible available reinforcements, prepositioned equipment, and enhanced command and control," the communique said.

In the communique, the alliance described China as a challenge to NATO's interests, security and values, and as a country that is seeking to undermine the rules-based international order.

NATO on Wednesday affirmed its commitment to continue supporting Ukraine, calling Russia an immediate and direct threat to European security, and welcomed Finland and Sweden into the alliance, a senior U.S. official also told reporters in a briefing.

The official said Turkey, which has lifted its initial objections to the two new members, did not ask for anything as part of the agreement to expand the alliance and the United States did not offer anything. Technical discussions were ongoing over Ankara's request for F-16 fighter jets, but the U.S. Congress has the final say about such arms sales, the official added.