In her first interview with international media, the mother of a man sentenced to a brutal death in Saudi Arabia has urged President Obama to call for her son's release.

Ali Mohammed al-Nimr was arrested in 2012 aged 17 for taking part in the Saudi Arabian protests during the Arab Spring in 2011. He was sentenced to death in May 2014; a punishment that would see him beheaded and then his decapitated body crucified.

A number of key political figures have called for al-Nimr's released, including Jeremy Corbyn during his speech at the Labour Party conference last month, but the US administration – an ally of Saudi Arabia – is yet to intervene.

Al-Nimr's mother, Nusra al-Ahmed, has now urged Obama to step in. In an interview with the Guardian, she said: "He [Obama] is the head of this world and he can, he can interfere and rescue my son...To rescue someone from harm, there is nothing greater than that...He would be rescuing us from a great tragedy."

A variety of charges are being held against al-Nimr, including attending a protest, using his phone to incite support for the demonstration and banditry. His family believe his harsh sentence is due to his uncle Nimr Baqr al-Nimr, a prominent Shiite cleric, having spoken out against the regime. He, too, has been given the death sentence, and is accused of "inciting sectarian strife".

Al-Ahmed says her son, now 21, has been tortured in prison and his teeth have fallen out. "For a month he was peeing blood," she told the Guardian. As he was under 18 at the time of his arrest, al-Nimr's sentence violates the UN Convention on the Rights of the Child, to which Saudi Arabia is a signatory.

A petitition urging the US government to negotiate for al-Nimr's release now has almost 98,000 signatures, and a State Department spokesperson has said the administration is "deeply concerned" by the case.

According to the BBC, an administration official said: "We call on the Government of Saudi Arabia to respect universal human rights and its international obligations as well as to ensure fair and transparent judicial proceedings that afford requisite fair trial procedures and safeguards in this and all cases.

"We have raised this case with the Saudi government including very recently, and addressed it in our 2014 Human Rights Report."

The Saudi Ambassador to the UN said he would not discuss al-Nimr's situation on BBC's Newsnight last week because the "legal process has not been exhausted".

In a press statement earlier this month, the Saudi UK Embassy said it "rejects any form of interference in its internal affairs and any impingement on its sovereignty or the independence and impartiality of its judiciary."