A British surgeon who died in detention in Syria was "in effect murdered" by the country's regime, a Foreign Office minister has said.

Hugh Robertson said there could be "no excuse whatsoever" for the treatment of Dr Abbas Khan, who was imprisoned by Syrian security forces while helping casualties of war.

Khan, 32, was seized by government troops in the rebel-held city of Aleppo in November 2012 after he entered the country on a humanitarian mission without a visa.

The Syrian security agency had promised his release this week but it was revealed Khan, an orthopaedic surgeon from Streatham, south London, had died in detention. News of his death was announced by his brother, Afroze Khan. In a starkly worded response, Robertson told the BBC: "There is no excuse whatsoever for the treatment that he has suffered by the Syrian authorities who have in effect murdered a British national who was in their country to help people injured during their civil war." Afroze Khan, 34, told the BBC: "My brother was going to be released at the end of the week. We were given assurance by the Syrian government. "My brother knew that. He was ready to come back home. He was happy and looking forward to being released." He added the family was angry at the Foreign Office for "dragging their feet" for more than a year.

The Foreign and Commonwealth Office in London said it was "urgently seeking" confirmation from the Syrian authorities about Dr Khan's fate.

An FCO spokesman said: "If these tragic reports are true, responsibility for Dr Khan's death lies with them and we will be pressing for answers about what happened.

"We have consistently sought consular access to Dr Khan and information on his detention, directly and through the Russians, Czechs and others."

In November Robertson made clear the British government's concerns about Khan's welfare and treatment but the efforts were ignored, the FCO said.

Khan's family lost contact with him for months after he went to Syria but his mother, Fatima, travelled to Damascus earlier this year to try to find out what had happened to him.

When she tracked him down, she found his weight had dropped below 5 stone (32kg) and he was barely able to walk. He told her he had been held for eight months in a dark, underground cell and tortured. She was told she could visit her son on Monday but when she arrived she was told he was dead, Afroze Khan told the BBC.

Last week Afroze Khan had warned that his brother was depressed and might want to harm himself.

A message on the Free Dr Abbas Khan Twitter feed said his life had been "taken meaninglessly". It added: "He was the best brother I could ever asked for and I know no one with a purer heart than him. His release was due to be this week."

Amnesty International said Khan's death reinforced the need for Britain to press the UN security council to refer the situation in Syria to the international criminal court.

Kristyan Benedict, Amnesty International's UK Syria campaign manager, said: "We don't know the full circumstances yet but this is yet another deeply troubling death in custody in Syria. "We know all too well that the torture of detainees is widespread and committed with impunity by the Syrian authorities, with detainees often crowded into vermin-infested cells, denied urgently needed medical treatment and even abused by medical staff.

"The UK government should denounce Dr Khan's death in the strongest possible terms and ensure that, no matter how long it takes, whoever is responsible is brought to justice."

More than 1,000 people are believed to have died in the custody of the Syrian security forces since the start of the crisis in the country in March 2011, according to Amnesty. Torture and ill-treatment of detainees, including children, is said to be widespread and committed by government forces and associated militias to extract information or "confessions". Detainees held in cramped, unsanitary conditions often suffer beatings, suspension by the limbs, being hung in a tyre, electric shocks and sexual abuse.

The Bradford West Respect MP George Galloway, who had been negotiating with the Syrian authorities to secure Khan's freedom, said he had been due to fly to Damascus on Friday to bring him home.

He said: "I think we will have to wait for clarification on how exactly he died but this is heartbreaking and devastating news for his family who have been working so hard for so long to secure his release. Particularly because his freedom had been agreed and he was due to return with me in the next few days." He added: "I have been in contact with the Syrian government many times, up to and including the president, the foreign minister, the justice minister and other ministers. "Last week I received a call from the foreign minister telling me that the president had asked him to contact me to come to Damascus to bring Dr Khan home before Christmas. "Obviously this had to be kept confidential but the family were kept fully informed. I was in the process of booking a flight for this Friday when I got the appalling news." Khan's former colleagues at the Royal National Orthopaedic Hospital in Stanmore, north-west London, said they were "very saddened" to learn of his death. He had been a specialist registrar in orthopaedic surgery at the hospital from September to October 2012. A spokesman said the hospital's "deepest sympathies" were extended to Khan's family.

Afroze Khan told London's Evening Standard newspaper: "It is just barbaric, it is medieval.

"It has been a whole year, a lot of ups and downs, but there was light at the end of the tunnel. We are just devastated. Words cannot describe it." Khan's family said he had been "accused of treating dying civilians [women and children] which has been classed as an act of terrorism", the newspaper reported.

All UK consular services in Syria have been suspended for some time and the FCO said it continued to advise against all travel to Syria.