Like most 6-year-olds, Jacob Abedini sat with a black pen in hand and excitedly sent out invitations to his seventh birthday party Tuesday. But the guest whose attendance is most important won't be there -- his father, American Pastor Saeed Abedini, who remains locked inside an Iranian prison because of Christian faith.

Abedini, a U.S. citizen and 34-year-old married father of two from Idaho, has been held in Iran for two and a half years after being accused of proselytizing in the Islamic republic. The American Center for Law and Justice and Abedini's family have pleaded with the U.S. and UN officials to secure his release -- including a meeting between the man's wife, Naghmeh Abedini, and President Obama earlier this year in Idaho that renewed hope for the family. 

As the U.S. conducts sensitive negotiations with Iran over its nuclear ambitions, the family is left wondering about the status of Abedini -- especially his young son Jacob.

During the 10 minute face-to-face meeting with Naghmeh and her children, Obama told the boy he would try to bring his father home in time for his birthday -- a promise that so far hasn't succeeded. 

Instead, Abedini responded to his son's invitation -- smuggled into the prison by a relative -- with a heartbreaking birthday letter written from his prison cell.

"I saw your beautiful birthday invitation that you had made me and I know how much you want me to be there on your birthday. Daddy loves you so much," Abedini wrote. "I long to be there for your birthday and to make this reunion happen, but my chains are keeping me from you."

"...That breaks my heart as your father," he wrote. 

Click here to read the entire letter

Abedini traveled to his native Iran in the summer of 2012 to visit family and work on the building and opening of an orphanage in the city of Rashit. On July 28, 2012, the Iranian Revolutionary Guard detained Abedini and ordered him to face criminal charges for his Christian faith. He was placed under house arrest and told to wait for a court summons, but on Sept. 26 he was taken from his family's home to the notorious Evin Prison, where he was placed into solitary confinement and subjected to regular beatings.

Prison doctors refused to treat Abedini for his wounds -- calling him "unclean" and an infidel -- and he remains in urgent need of medical care, according to the ACLJ.

A petition has been started in Saeed’s name -- with more than 610,000 signatures -- and Secretary of State John Kerry, the White House and Congress have all publicly called for his release.

In a statement released by the State Department, Deputy Spokesperson Marie Harf said:

"We are doing everything we can to secure the release of Pastor Saeed Abedini and the other U.S. citizens detained or who went missing in Iran, including raising these cases directly with Iranian officials on the sidelines of the nuclear talks every time we meet. We continue to call for Pastor Abedini's immediate release, as well as the immediate release of detained U.S. citizens Jason Rezaian and Amir Hekmati, and for the Iranian government to work cooperatively with us in locating Robert Levinson, so that all can be returned to their families as soon as possible."

Although Abedini was born a Muslim, he converted to Christianity in 2000 while living in Iran. He became active in the nation's network of home churches, which were at the time tolerated by Tehran. His supporters say he helped establish as many as 100 home-based Christian churches prior to a crackdown in 2005 which prompted him and his family to flee to the U.S. He returned in 2009, and was detained and threatened with execution for apostasy, although he was ultimately freed under the condition he not evangelize.

National Security Council spokeswoman Bernadette Meehan did not provide any updates on the Abedini case when contacted Monday. At the National Prayer Breakfast in Washington last month, Obama underscored his commitment to securing the pastor's release.

"Last year we prayed together for Pastor Saeed Abedini detained in Iran since 2012," the president said. "And I was recently in Boise, Idaho and had the opportunity to meet with Pastor Abedini’s beautiful wife and wonderful children and to convey to them that our country has not forgotten Brother Saeed and that we are doing everything we can to bring him home."

Obama's promise, however, has not yet been fulfilled and the administration has released few details about the status of any negotiations.

"Pastor Saeed languishes in a filthy and violent prison in Iran -- not because he poses any threat or violated any law -- but because he peacefully gathered to pray with other Christian believers almost 15 years ago," Jordan Sekulow, ACLJ's executive director, said Monday.

"As the U.S. government sits down with Iran in Geneva it is imperative that our diplomats carry out President Obama's promise to young Jacob and secure Pastor Saeed's release in advance of any formal agreement with Iran," Sekulow said.

Naghmeh Abedini told FoxNews.com that her son's birthday Tuesday will be an especially painful reminder of his father's absence. 

"It will be hard watching Jacob wake up tomorrow morning with so much hope and anticipation of seeing his father," she said. "Sadly, I will have to tell him that daddy will have to miss yet another birthday."

"But I will be able to give him a piece of Saeed," Abedini said of the letter. "I do hope that this is the last birthday that the kids will have to celebrate with so much pain in the hearts and tears streaming down their faces."

FoxNews.com's Cristina Corbin contributed to this report.