September 4, 2015 (1:10 p.m. EDT): On September 3, the United States government issued a Travel Warning for Turkey stating "U.S. citizens traveling to or residing in Turkey should be alert to the potential for violence."

The warning refers to terrorist attacks on U.S. interests in the country, as well as at sites frequented by foreign tourists. Visitors to the country are encouraged to remain aware of their surroundings and maintain a high level of vigilance.

(5:27 p.m. EDT) -- Celebrity Cruises has become the second cruise line to cancel a call at Istanbul. Today the line said Celebrity Reflection's August 31 departure will skip its overnight visit there. Costa Cruises pulled its ships out of Istanbul through 2015, in reaction to concerns about safety following a deadly terrorist attack in Turkey's largest city earlier this month. It also canceled calls at Izmir.

In a statement, Celebrity Cruises said, "We have been closely monitoring the security situation in Istanbul, Turkey, and have decided, in an abundance of caution, to cancel Celebrity Reflection's overnight call to Istanbul on Friday, September 4 and Saturday, September 5."

This isn't Celebrity's first itinerary change involving Turkey in August. The line also canceled Celebrity Equinox's overnight call at Istanbul on Monday, August 24. The ship -- which arrived in port that morning so passengers from the previous sailing could disembark -- left Monday night at 6 p.m. instead.

For passengers onboard this Reflection cruise, the line will replace the Istanbul call with an overnight in Athens.

At this time, the U.S. Department of State has issued a “security message” in response to the shooting incident near the U.S. Consulate earlier this month. "Due to a record of past terrorist attacks in Turkey and a continuing threat from both transnational and indigenous groups, the threat level for the possibility of further terrorist attacks against U.S. citizens and interests remains critical." On its website, the U.K.'s Foreign and Commonwealth Office, which offers similar consular services to its citizens, also cautions citizens to travel carefully.

In the next few weeks, other cruise lines slated to visit Istanbul, an extremely popular turnaround port in summer, include Norwegian, Princess, Holland America, MSC Cruises, Silversea and Windstar. So far, Norwegian, Silversea, Windstar, Cunard, Seabourn and MSC have confirmed they have not yet made any changes. Cruise Critic has reached out to the other lines for an update. We'll continue to keep you posted.

--By Gina Kramer, Associate Editor