YEREVAN/BAKU (Reuters) - At least 16 military members and several civilians were killed on Sunday in the heaviest clashes between Armenia and Azerbaijan since 2016, reigniting concern about stability in the South Caucasus, a corridor for pipelines carrying oil and gas to world markets. The clashes between the two former Soviet republics, which fought a war in the 1990s, were the latest flare-up of a long-running conflict over Nagorno-Karabakh, a breakaway region that is inside Azerbaijan but is run by ethnic Armenians. Nagorno-Karabakh said 16 of its servicemen had been killed and more than 100 wounded after Azerbaijan launched an air and artillery attack early on Sunday. Armenia and Nagorno-Karabakh declared martial law and mobilised the male population. Azerbaijan, which also declared martial law, said its forces responded to Armenian shelling and that five members of one family had been killed by Armenian shelling. It also said its forces had seized control of up to seven villages. Nagorno-Karabakh initially denied that but later acknowledged losing “some positions” and said it had suffered a number of civilian casualties, without giving details. The clashes prompted a flurry of diplomacy to reduce the new tensions in a decades-old conflict between majority Christian Armenia and mainly Muslim Azerbaijan, with Russia calling for an immediate ceasefire and another regional power, Turkey, saying it would support Azerbaijan. President Donald Trump said on Sunday the United States would seek to end the violence. RELATED COVERAGE Trump says U.S. will seek to stop violence between Armenia, Azerbaijan Factbox: Nagorno-Karabakh - old tensions erupt again into violence See more stories “We’re looking at it very strongly,” he told a news briefing. “We have a lot of good relationships in that area. We’ll see if we can stop it.” The U.S. State Department condemned the violence in a statement, calling for an immediate halt to hostilities and any rhetoric or other actions that could worsen matters. U.S. Democratic presidential nominee and former Vice President Joe Biden said in a statement that hostilities could escalate into a wider conflict and urged the Trump administration to push for more observers along the ceasefire line and for Russia “to stop cynically providing arms to both sides.” Pipelines shipping Caspian oil and natural gas from Azerbaijan to the world pass close to Nagorno-Karabakh. Armenia also warned about security risks in the South Caucasus in July after Azerbaijan threatened to attack Armenia’s nuclear power plant as possible retaliation. Nagorno-Karabakh broke away from Azerbaijan in a conflict that broke out as the Soviet Union collapsed in 1991. Although a ceasefire was agreed in 1994, after thousands of people were killed and many more displaced, Azerbaijan and Armenia frequently accuse each other of attacks around Nagorno-Karabakh and along the separate Azeri-Armenian frontier. Slideshow ( 5 images ) In Sunday’s clashes, Armenian right activists said an ethnic Armenian woman and child had also been killed. INTERNATIONAL DIPLOMACY Armenia said Azeri forces had attacked civilian targets including Nagorno-Karabakh’s capital, Stepanakert, and promised a “proportionate response”. Slideshow ( 5 images ) “We stay strong next to our army to protect our motherland from Azeri invasion,” Armenian Prime Minister Nikol Pashinyan wrote on Twitter. Azerbaijan denied an Armenian defence ministry statement that said Azeri helicopters and tanks had been destroyed, and accused Armenian forces of launching “deliberate and targeted” attacks along the front line. “We defend our territory, our cause is right!” Azerbaijan’s president, Ilham Aliyev, said in an address to the nation. Turkey said it was talking to members of the Minsk group, which mediates between Armenia and Azerbaijan. Russia, France and the United States are co-presidents. Russian President Vladimir Putin spoke by phone to Pashinyan but no details of the conversation were available, and Turkish President Tayyip Erdogan spoke to Aliyev. Erdogan, promising support for traditional ally Azerbaijan, said Armenia was “the biggest threat to peace in the region” and called on “the entire world to stand with Azerbaijan in their battle against invasion and cruelty.” Pashinyan hit back, urged the international community to ensure Turkey does not get involved in the conflict. The European Union and the Organization for Security and Cooperation in Europe (OSCE) urged both sides to stop military actions and return to negotiations, as did Pope Francis. At least 200 people were killed in a flare-up of the conflict between Armenia and Azerbaijan in April 2016. At least 16 people were killed in clashes in July. Additional reporting by Tuvan Gumrukcu in Ankara and Diane Bartz in Washington; Writing by Gabrielle Tétrault-Farber and Vladimir Soldatkin; Editing by David Evans, Daniel Wallis and Peter Cooney