Story highlights Dustin Johnson and Zach Johnson withdraw from golf tournament in South Korea

American golfers worried about recent tensions on the Korean Peninsula

Tournament sponsor insists that the event can safely go ahead this week

Ballantine's Championship is co-sanctioned by European and Asian tours

The sponsor of a top golf tournament in South Korea insists it is safe to go ahead despite two American players pulling out due to security fears.

U.S. Ryder Cup stars Dustin Johnson and Zach Johnson withdrew from the Ballantine's Championship Monday, citing concerns over recent tensions between North and South Korea.

"They have reached this decision following perceived unrest on the Korean Peninsula and can no longer assure their commitment to this year's event," Ballantine's said in a statement on the website of the European Tour, which co-sanctions the tournament with the Asian Tour.

Now held at the Blackstone course at Icheon, 60 kilometers south of the capital Seoul, the €2,205,000 ($2.85 million) event was the first staged in South Korea in 2008 on the island of Jeju.

"Ballantine's and the European Tour would like to reiterate that the tournament is going ahead as planned with a full field of European, Asian and Korean PGA Tour members," the statement added.

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"Ballantine's has been kept fully up to date on relevant issues and the European Tour has also been liaising with key decision makers, including the Foreign Office, who have not warned of any increased risk to those traveling to or within the country.

World No. 30 Zach Johnson, who won the 2007 Masters, said he did not want to take the risk following North Korea's recent threats against its neighbor and its move to test missiles and restart its nuclear program.

"It was one of those things where the women in my life really didn't feel like it was necessary for me to go," the 37-year-old from Iowa told the Des Moines Register , referring to his wife, mother and young daughter.

"The posture and rhetoric that is taking place right now makes it not worth it. And it probably is just posture and rhetoric. However, I don't want to take the chance."

World No. 19 Dustin Johnson, who played at the Korean event in 2011, had similar concerns.

"I feel bad for the tournament," said the 28-year-old, whose partner is Paulina Gretzky, the model daughter of ice hockey legend Wayne Gretzky. "They were gracious and understanding. Hopefully it will work out in the future."

The tournament, which starts on Thursday, will feature three former major champions in South Africa's world No. 6 Louis Oosthuizen, South Korean veteran Y.E. Yang and Britain's Paul Lawrie.