NATO Secretary General Anders Fogh Rasmussen on Friday pledged to strengthen the shield along the border with Russia, and urged allies to increase their military budgets.

Rasmussen, who paid a one-day visit to Bulgaria, said at a joint press conference with the country's Foreign Minister Kristian Vigenin that "We are taking legitimate steps to reinforce our support to Allies, from the Baltic to the Black Sea."

NATO has stepped up practical and political support to Ukraine, to bolster its defense capacity within the framework of their partnership, he said.

"We are reaching out to Moldova, Georgia and other long-standing partners across the region to strengthen our cooperation," Rasmussen said.

"Bulgaria has a key role in the region, and this region is key to NATO," he said.

He added NATO made clear to Russia that "this can no longer be business as usual," while keeping open the diplomatic channels of communication.

"It is urgent that Russia ends its destabilization of Ukraine" and rebuilds trust, Rasmussen said.

Earlier on Friday, Rasmussen said after talks with Bulgarian President Rosen Plevneliev that NATO has to be kept strong.

"Above all, we must stop the decline of our defense budgets and start reversing the trend, and work even closer together to acquire the critical modern capabilities that we need," Rasmussen said.

"Today, we know that we cannot simply take our security for granted, and that our world is more unpredictable than it appeared, " he said. So, he expected all allies to play their part in keeping NATO the strongest Alliance in history, he added.

Rasmussen also met with Bulgarian Prime Minister Plamen Oresharski.

Bulgaria is making efforts to contribute to NATO operations, Oresharski told Rasmussen.

"We are ready to do so in the future as well," Oresharski said according to the government information service (GIS).

Oresharski stressed the need to improve defense expenditure in Bulgaria, and further modernization of the army and adaptation to NATO standards remained a priority.

Regarding the Ukraine crisis, Oresharski pointed out that "Bulgaria remained hopeful that the conflict would be resolved through diplomatic means," GIS said.

During the meeting between Plevneliev and the NATO secretary-general, the nationalist Ataka party that generally supports the government in the parliament, held a protest against Rasmussen's visit.

Ataka's leader Volen Siderov said his party was against Bulgaria's involvement in any military actions or support of military operations.