Istanbul  - Officials in Turkey and Azerbaijan frequently refer to the two countries - bound together by language and culture and, more recently, by oil and gas pipelines - as "one nation-two states."

But this slogan is being put to the test these days, following the recent signing between Turkey and Armenia of protocols to restore diplomatic relations and open up their borders, something Azerbaijan is strongly opposed to.

Observers warn that the tension between Turkey and Azerbaijan could not only undermine the two neighbors' relations, but also jeopardize some of the energy deals the two countries are involved in, particularly the Nabucco pipeline project, which is designed to ease Europe's dependence on Russian gas.

The historic protocols make no mention of Nagorno-Karabakh, a mountainous Azeri territory occupied by Armenian forces since 1994.

Turkey first closed its border to Armenia in response to its occupation of the territory and Azeri officials have said Ankara's relations with Yerevan should not be restored until the frozen Nagorno-Karabakh issue is settled.

The reaction in Baku to the deal signed by Turkey and Armenia - which must still be ratified by the two countries' parliaments to take effect - has been particularly strong. Turkish flags were taken down at official buildings and monuments in the Azeri capital, Baku.

Azerbaijani President Ilham Aliyev, speaking at a nationally televised cabinet meeting on October 16, suggested his country might stop selling Turkey natural gas at a discounted price.

"It is not a secret to anyone that for many years Azerbaijan has been selling its gas to Turkey for one-third of market prices," Aliyev said.

"What state would agree to sell its natural resources for 30 percent of world market prices, especially under current conditions? This is illogical," Aliyev added.

"Azerbaijan is looking at the opening of the Turkey-Armenia border as a betrayal.

"We are looking at Turkey as our main partner in the region and as Armenia as our main enemy. The reaction is natural," says Vafa Guluzade, former foreign affairs advisor to the previous Azeri president, Haydar Aliyev.

"The situation right now is that the Turkish-Azeri relationship is threatened by the threat of full destruction. I'm not talking on the level of the people - I'm talking on the level of the leadership," he says.

Armenian forces in Nagorno-Karabakh and some surrounding districts today control some 20 percent of Azerbaijan's territory. The United Nation estimates that there are 690,000 internally displaced persons from the territory in Azerbaijan.

Although Azerbaijan and Armenia are engaged in ongoing talks, co- chaired by the United States, Russia and France, aimed it resolving the issue, although they have made little progress.

Experts warn that without any movement on the Nagorno-Karabakh front, the opening of the Turkish-Armenian border could lead to an even stronger reaction from Baku, particularly on energy issues.

Azerbaijan sits on top of large oil and gas reserves and has emerged in recent years a major player in the energy field. Turkey, meanwhile, is positioning itself to become a major oil and gas transit country - a plan that, to a large extent, depends on access to Azeri energy resources.

"If those countries which are genuinely interested in the Nabucco project influence Armenia so that it steps back from the occupied territories and both Azerbaijan and Turkey will be satisfied, then this will be remembered as only an episode in Turkey-Azerbaijan relations," says Ilgar Mammadov, a political analyst based in Baku.

"If not, then the relationship between Turkey and Azerbaijan will worsen and projects like Nabucco will be undermined."

Turkish officials have been making efforts to appease Baku, making it clear that they will not move forward on renewing ties with Armenia until the Nagorno-Karabakh.

"Nothing can change the fact that Turkey will always stand by Azerbaijan, and Turkey's position on Azerbaijan's territorial integrity and its territories under occupation," Turkish Foreign Minister Ahmet Davutoglu said during a Thursday visit to the Azeri capital.

But Guluzade, the former Azeri foreign affairs advisor, says Baku is looking for stronger reassurances on the issue.

"We need something more real in our hands. Armenia has signed protocols under the auspices of The United States, Russia and France. We only have words. That's not enough," he says.

Some Turkish officials, though, have started to express frustration with Azerbaijan's strong response to Ankara's budding détente with Yerevan.

"The Azeri reaction is very disappointing. We have a difficulty understanding these reactions and the forms and ways in which they are happening," says Suat Kiniklioglu, a member of the governing Justice and Development Party (AKP) and spokesman for the parliament's foreign affairs committee.

Huseyin Bagci, a professor of international relations at Ankara's Middle East Technical University, says that in the long run the opening of the Turkey-Armenia border will also facilitate the solving of the Nagorno-Karabakh issue.

"Azerbaijan has to realize that discussion of the Nagorno- Karabakh issue can only happen if the Turkey-Armenia border opens," he says.

In the meantime, without a resolution to the conflict betwee Azerbaijan and Armenia, Ankara and Baku may find the "one nation-two states" slogan being further tested, Bagci says.

"It's a nationalist slogan, it's nice to hear, but it's never been the case. We have never been one nation," he says. (dpa)