Iranian officials say they have confirmed the country's first two cases of MERS, a deadly virus first reported two years ago in Saudi Arabia, its neighbour on the western side of the Gulf.
Middle East Respiratory Syndrome Corona-Virus (MERS) causes coughing, fever and sometimes fatal pneumonia, killing an estimated 30 percent of those who are infected.
There is no vaccine or specific treatment for MERS, which has killed more than 175 people in Saudi Arabia and spread throughout the region, also reaching as far as Malaysia, Greece, Lebanon and the United States.
"Four suspected cases of new corona virus infection were observed in a family in the province of Kerman. Two of these cases were confirmed in two sisters," said Mohammad Mahdi Gouya, the director-general of communicable diseases at the Iranian Health Ministry's Centre for Diseases Control and Prevention.
"One of the sisters is in critical condition and the other is currently receiving treatment under special circumstances," the ministry's website quoted him on Monday as saying.
A recent upsurge of infections in Saudi Arabia is of concern because of the influx of pilgrims from around the world expected in July during the Muslim fasting month of Ramadan.
Gouya also said that Tehran had dispatched trained medical teams to Saudi Arabia, where they studied MERS cases among Iranian Hajj pilgrims, according to Iran's Press TV.
Arrangements were being made for Iranian pilgrims to undergo medical check-ups after they return home, he added.
MERS is a virus from the same family as SARS, or Severe Acute Respiratory Syndrome, which killed about 800 people worldwide after it first appeared in China in 2002.