NATO invited the Balkan nation of Montenegro to become its 29th member, agreeing Thursday to expand for only the seventh time in its history despite Russia's angry objections.

The decision is still subject to formal approval by the U.S. Senate, the alliance's 27 other national parliaments and Montenegro's parliament.

NATO Secretary-General Jens Stoltenberg said it was the "beginning of a new secure chapter" in the former Yugoslav republic's history. He and U.S. Secretary of State John Kerry hailed the move as proof that NATO is committed to its "open door policy" of expansion despite opposition from Russia or any other country.

"Montenegro's accession underscores once again our determination to be able to make membership decisions that are free from outside influences and underscores our resolve to stand together against any kind of threat," Kerry said.

Montenegrin Prime Minister Milo Djukanovic attended the signing of an accession protocol at NATO headquarters in Brussels. He said his country, bombed by NATO warplanes 16 years ago, would stand "shoulder to shoulder" with the other members of the U.S-led alliance.

"You can count on us at any time," Djukanovic said.

Russia has accused NATO of trying to encircle it, and Russian allies like Serbia have vowed to do what's necessary to defend its national security and interests.

"Yet another NATO attempt to change the military political landscape in Europe, especially in light of the alliance's course to restrain our country, inevitably affects Russian interests and forces us to react accordingly," Russian Foreign Ministry spokeswoman Maria Zakharova said Thursday.

Sergei Zheleznyak, a prominent member of the Russian parliament, said his country will have to alter its relations with Montenegro, which is historically close to Russia, if it joined NATO without holding a national referendum.

"We would have to change our policy in regard to this friendly country," Zheleznyak said. "If NATO military infrastructure were placed there, we would have to respond by limiting our contacts in economic and other spheres."

Other Russian officials have said Russia could ban some imports from Montenegro and levy other trade sanctions.

The signing ceremony at NATO headquarters for Montenegro's membership invitation coincided with the start of a NATO foreign ministers' meeting designed to set the stage for the alliance's summit in Warsaw this July.

NATO now faces simultaneous security challenges from Russia and from armed. Extremist movements in the Middle East and North Africa, and has been progressively devising a response. Stoltenberg said ministers agreed Thursday to send an assessment team to Iraq to explore the possibility of NATO training Iraq's military in the country to help it better fight Daesh. NATO already has trained hundreds of Iraqi officers in Jordan.

Stoltenberg said NATO is also considering aiding the U.S.-led coalition combating Daesh by supplying AWACS command and control aircraft that could fly over Turkey or in international airspace and peer into Iraq and Syria. And he said Libya's new U.N.-brokered government will send experts to Brussels to explore how NATO might help Libya rebuild its shattered defense and security apparatus to fight a local Daesh affiliate.

"NATO is already doing a great deal with partners and for partners," said Stoltenberg. "But we can and should do more. Because projecting stability in our neighborhood is also about preserving security here at home."

Since NATO's creation in 1949 as a bulwark of the West's Cold War defenses against the Soviet Union, it has grown from 12 founding members to absorb most of the Kremlin's former allies in the communist Eastern Bloc. NATO last added new members in 2009, when Albania and Croatia joined.

Asked by reporters how long it will take for Montenegro to become a fully-fledged member, Stoltenberg said he couldn't predict how fast legislators in NATO member nations will act, but that ratification of the accession protocols took about a year in the last expansion round.

"I expect we will soon see 29 allied flags flying outside the NATO headquarters," Stoltenberg said. Until ratification is complete, he said Montenegro is guaranteed a "seat at the table" at alliance proceedings as an observer.

Montenegro would be among NATO's smallest members, boasting active-duty armed forces of only 2,000. But Stoltenberg said it has already contributed to NATO-led missions in Kosovo and Afghanistan, and that the document signed Thursday "shows once again that NATO's door remains open" to countries like Georgia, Macedonia and Bosnia, which also aspire to become members one day.

"Montenegro is a signal that the alliance is not giving up on the enlargement process and that Russia holds no veto on the accession of an aspirant country," said Bruno Lete, senior program officer at the German Marshall Fund, a Brussels think tank.

Another U.S.-based NATO expert, though, said for the alliance to open its ranks to Montenegro hardly constitutes a "brave challenge to Russia" and that NATO and the Kremlin alike are exaggerating the significance.

"Montenegro is joining NATO because it is small enough and far away enough from Russia's borders to be a relatively safe decision for NATO governments," said Jorge Benitez of the Atlantic Council in Washington.