The Foreign Office said on Friday it was "disappointed and saddened" by a US court's decision to sentence Dr Aafia Siddiqui to 86 years in jail, and would work to bring her home.
The New York court found Aafia Siddiqui guilty of the attempted murder of US officers in Afghanistan in a case that sparked outrage in Pakistan.
"We are disappointed and saddened by the sentence but the government has not given up," foreign ministry spokesman Abdul Basit told AFP, hours after Siddiqui's family vowed to launch a "movement" to get her released from jail.
The tough jail term sparked immediate outrage in Siddiqui's hometown of Karachi and in Lahore, where hundreds of activists demonstrated, chanting anti-US slogans.
"We are continuing efforts to get Aafia Siddiqui repatriated to Pakistan," the foreign ministry spokesman said.
The government has come under pressure at home, particularly among right-wing groups, as well as from Siddiqui's family for failing to get her released.
Asked under what circumstances Siddiqui could return to Pakistan, Abdul Basit said US President Barack Obama could pardon her, or an agreement could be reached for her to serve at least part of her sentence in Pakistan.
Federal Interior Minister Rehman Malik said the government will surely bring back Dr Aafia Siddiqui but it needs time to do so.
In a statement, Pakistan's ambassador to the United States Hussain Haqqani said the government took every possible step for the safe release of Dr Aafia. Haqqani added that Pakistan will continue efforts for the safe return of Aafia.
PML-N chief Mian Nawaz Sharif has expressed his grief over the sentence given to Dr Aafia Siddiqui.
In a special message to Dr Aafia's family, Nawaz said he had raised the issue of Dr Aafia's release in his meeting with US special envoy Richard Holbrooke. The PML-N chief said he would make all efforts to secure Dr Aafia's release. He also added that the entire nation was praying for her safe return.
JUI-F Chief Molana Fazalur Rehman has cancelled his visit to America as a protest against the decision regarding Doctor Aafia.
Talking to media outside the parliament house he said that the punishment of Doctor Aafia has added to the list of American crimes.
Protests against Aafia's sentence
Fauzia Siddiqui told reporters in Karachi that all of Pakistan would agitate to get her sister freed. "I was alone... when I started the campaign to release my sister, but from now on it will be the Aafia movement as the whole nation is with me," she said.
Around 200 activists from Jamaat-e-Islami and various right-wing groups gathered outside Siddiqui's Karachi home overnight as police went on full alert to guard against possible violence. The crowd chanted slogans including "Down with America" and "Allahu akbar" (God is greater) soon after news of the sentence filtered through.
"We have declared a high alert and deployed maximum police force in the city to stop possible violence and ensure that no private property is damaged during future protests," the city's police chief Fayyaz Leghari told AFP.
In Lahore, around 200 activists from Islami Jamiat Talaba, the student wing of Jamaat-e-Islami, burnt tyres and shouted anti-US slogans after the sentencing.
US human rights activists condemn sentence
A New York-based human rights group has condemned as "unfair and unjust" the sentence handed down to Aafia Siddiqui, a Pakistan neuroscientist, saying she had never caused harm to anyone.
"This sentence is not only unjust because of its harshness to Dr Siddiqui - but also because of its impact on her two small children in Pakistan who may never see their mother again," the International Justice Network, attorneys for Siddiqui's family, said in a statement after Thursday's tough sentence imposed US District Judge Richard Berman.
"But the greatest injustice of all is that those who are responsible for the kidnapping, disappearance, and abuse of Dr Siddiqui and her children without cause have yet to answer for their actions.
"While today's sentence concludes a shameful chapter in American history, it is only a matter of time before the truth about what has been done to Dr Siddiqui, her family, and the thousands of other innocents who have been disappeared is revealed. Importantly, despite today's sentence and all the injustices which she has endured, Dr Siddiqui has consistently made clear that she does not support any acts of violence being taken in her name," the statement added.
"The International Justice Network stands in solidarity with the international community in condemning this unfair and unjust result in Dr. Siddiqui's case."
"This case is not over. This is just the beginning," Tina Foster of the International Justice Network told reporters, adding that the real importance of the case, her group believes, is that it draws attention to thousands of disappearances in Pakistan.