A top security official at the US embassy in Nairobi is said to have warned that work stoppage by guards "put Kenyan and American lives at risk".
A Washington-based NGO said on Tuesday that it obtained the January 28 email sent to heads of the embassy private guard force and other recipients.
The unnamed security officer was condemning the work stoppage at the embassy's Gigiri complex by hundreds of guards employed by Aegis-KK Security, a Kenyan subsidiary of the Canada-based private firm GardaWorld.
The Project on Government Oversight (Pogo) watchdog founded in 1981 added that it obtained records showing that about 400 day-shift security workers at the US embassy had begun a sit-in and work stoppage to protest non-payment of wages and benefits.
The company was hired by the State Department in 2013 under a multi-year contract, currently valued at more than $35 million (Sh3.5 billion), Pogo said.
KK employees provide security to the embassy and the official residences of US diplomats.
Hundreds of guards have been suspended, while untrained replacements have been brought in as part of an effort to close the gap in staffing levels, Pogo said in a report it posted on The Daily Beast, a US online news site.
The organisation attributed the information to an unnamed representative of the striking workers.
According to the report, the guards' representatives -- Charles Waweru, Abednego Mutua and Charles Oluoch -- said that when they met KK Security management on January 28, consensus on the contentious issues was not reached.
The three said the employer resorted to intimidation and harassment.
"After the meeting, the company opted to initiate disciplinary proceedings against the guards with a view to harassing and intimidating them in order to continue withholding their overtime arrears amounting to more than Sh1 billion," the guards said in a statement.
According to the guards, 358 of their colleagues have been dismissed since then.
"The guards have appealed the summary dismissals with a view to seeking reinstatement and payment of their dues," the statement went on.
In response to Sunday Nation queries, the embassy said the dispute is between the guards and their employer "and not the State Department or the US embassy in Nairobi".
"The department looks forward to an amicable resolution between the two parties," it said.
The embassy added that its contract with the guards includes contingency plans.
"In Nairobi, there is a contingency plan and it was executed. Beyond that, the department cannot comment on the details of our security procedures. The department is in regular communication with Aegis-KK Security," the embassy said.
KK Security could not be reached for comment as its phones went unanswered.
The embassy security work stoppage began less than two weeks after al-Shabaab terrorists killed 21 people at DusitD2 complex in Nairobi.
Pogo cited an email from a State Department official warning that the attack showed "there is a real threat of terrorism".
The NGO quoted the official's email: "If there is another attack, it is a question of when."
An al-Qaeda attack on the US embassy at the junction of Moi and Haile Selassie avenues in 1998 left more than 210 people dead.
After the bombing, the embassy was moved to Gigiri.
Pogo said the labour dispute "has close parallels to the deteriorating security at the State Department's compound in Benghazi, Libya, before the 2012 attack that killed US ambassador Christopher Stevens and three other Americans".
"The State Department-convened Accountability Review Board on Benghazi noted a labour dispute preceded the terrorist attacks," it said.
US embassy guards employed by Aegis KK-Security have battled compensation issues since March 2013.
The NGO said a court awarded the guards a portion of the money owed to them by Aegis KK-Security.
According to Pogo, the guards' union says the employees have only received a portion of what they were awarded.
The union says that over the years, its members have been dismissed and disciplined for demanding their rights.
"Guards at the embassy seem to earn more than the average Kenyan," Pogo says in its Daily Beast posting. "But they work 12-hour shifts: eight hours at regular pay, with four hours overtime."