By Hasmik Mkrtchyan

YEREVAN (Reuters) - Armed men holed up at a police station in Armenia's capital Yerevan took several doctors hostage on Wednesday, ratcheting up tension in their 10-day standoff with security forces.

The group of 30 gunmen seized the police station on July 17, killing a police officer, wounding two others and taking hostage nine officers.

They have demanded the release of jailed opposition politician Jirair Sefilian and the resignation of President Serzh Sarksyan.

After negotiations with security forces, they released four remaining captives on Saturday, but refused to surrender.

One policeman and four gunmen were wounded in a shootout last night, police said. A policeman and two gunmen were taken to hospital, while two wounded gunmen stayed at the police station.

"The armed group took hostage the doctors sent to the police station to give medical aid to the wounded gunmen," the police said in a statement.

Armenia's national security service called on the gunmen to lay down their arms, free the hostages and surrender. It set no deadline.

Any police operation to storm the seized station would be complicated. Dozens of people are gathering nearby every day, demanding that bloodshed be avoided.

The U.S. Embassy said it was worried by the situation.

"We are especially concerned by media reports today that medical personnel have been detained in the Erebuni Police building," the embassy said in a statement. "Such actions are unacceptable, and if true, we call for the immediate and safe release of these individuals."

The gunmen said the doctors had been asked to stay rather than taken hostage.

"They just want the doctors to be there permanently to help the wounded guys," Alex Yenikomshian, a member of the opposition Founding Parliament movement, told reporters.

Their jailed leader, Sefilian, is accused by the ex-Soviet state's authorities of plotting civil unrest. Sefilian was jailed in June over allegations of illegally possessing weapons.

A former military commander, Sefilian has accused Armenian leader Sarksyan of mishandling a long-running conflict between Armenian-backed separatists in the Nagorno-Karabakh region of Azerbaijan and Azeri forces.

A Moscow-brokered ceasefire halted four days of violence in the South Caucasus region in April, the worst flare-up in years. But sporadic shooting persists at night and some deaths have been reported.

(Writing by Margarita Antidze; Editing by Dmitry Solovyov, Larry Kings)