South Korean actor Song Joong Ki has been setting female hearts aflutter with his suave turn as an all-action special operations soldier in the hit drama Descendants Of The Sun.

Even his A-list co-star Song Hye Kyo, who plays his love interest Kang Mo Yeon, confesses she has felt her heart palpitate when watching her character being wooed by his Captain Yoo Si Jin.

Recalling a memorable line in a scene where the officer seizes the initiative to kiss Dr Kang, she says: "There are many great lines from the drama. I think many great lines came from Yoo Si Jin, not from my character.

"'Should I confess my love or apologise?' That was the line he said (after the kiss) that made my heart flutter.

"We wrapped up filming in December and the drama started to air in February. Since we didn't watch the end product during filming, I watched the drama while thinking of myself as Kang Mo Yeon. My heart fluttered when I heard that line."

The 34-year-old actress and Song Joong Ki, 30, were in Hong Kong on Tuesday to attend a press event for Descendants.

The stars were promoting the drama that will air on new Hong Kong channel ViuTV. When they arrived at the airport, they were greeted by hundreds of fans, according to reports.

In Singapore, the 16-episode series is available for free on K-drama streaming site viu.com and its app on smart devices.

But promoting the 13 billion won (S$15 million) drama hardly seems to be necessary.

Descendants - which superimposes an old-fashioned romance drama on a war-themed action thriller and imbues it with comic elements - has garnered top ratings across the region.

In Seoul, where episodes 11 and 12 aired last week, it scored ratings above the 30 per cent share of the audience, surpassing the numbers of another Korean global hit, My Love From The Star, in 2013.

It received more than two billion views on the Chinese streaming site iQiyi. The first four episodes of the drama were watched more than 300 million times on iQiyi and, after eight episodes, the drama broke the one-billion milestone.

In Hong Kong and Singapore, Descendants is the most-watched show on the Viu online service. Last month, more than 60 per cent of Viu's viewers in Singapore and Hong Kong tuned in to watch it.

Song Hye Kyo found the script, which was filmed in Greece standing in for a fictional war-torn country called Urk, to be a page- turner when she first received it.

She says: "The relationship between a doctor and a soldier intrigued me.

"When I read the script for the first time, I found I could read it as fast as if it were a comic book. That's when I realised it would be a fun drama that many people would fall in love with."

According to reports, screenwriter Kim Eun Sook - the creator of successful dramas such as The Inheritors (2013), Secret Garden (2010) and Lovers In Paris (2004) - reworked the script by Kim Won Suk, another screenwriter, beefing up the love story.

Indeed, Descendants may be chock-a-block with action - as Captain Yoo engages in hand- to-hand combat with North Korean soldiers, rescues hostages from terrorists and tangles with nefarious gunrunners - but Song Joong Ki believes that the drama is ultimately a romance.

He says: "You can see different kinds of scenes in the drama - there's action, there're earthquakes. But we still consider it a romantic drama.

"There are many couples like me and Hye Kyo as well as (the second leads) Jin Goo and Kim Ji Won in the drama. I hope the viewers would focus on the cute stories of those couples."

Between the romances and gunfights, the cast also has plenty of time for comedy, which often had the stars in stitches while the camera was rolling.

Song Hye Kyo says: "The characters aren't very serious at times. So you'll see funny parts here and there. Joong Ki and I both laughed a lot. We had to stop filming sometimes because we were laughing too hard. I remember how bad I felt for the crew at the shoot."

Perhaps part of the drama's success can be attributed to the adequate time given to the cast and crew to finish the production.

Typically, Korean dramas go to air even as filming is ongoing. Descendants, in contrast, wrapped up filming in December before it premiered in South Korea and China in February.

Song Joong Ki explains that an advantage of this arrangement is that there was time for discussions without the pressure of a tight filming deadline looming over cast and crew.

"We had enough time to communicate and work with one another. I liked the fact that we had a more relaxed setting. That's what I liked the most about doing this drama," he says.

He should be pleased that he fought for and scored the role of Captain Yoo.

Mr Kim Woo Taek, chief executive of New Entertainment World, which produced Descendants, has said there were other "superstar candidates" for the role.

He added that Song, who had previously starred in television dramas such as Sungkyunkwan Scandal (2010), "has a relatively weak image", which is the opposite of the manly Captain Yoo.

Of course, fans of Descendants - especially female ones - would disagree, now that they have seen Song being all alpha male and topless on a few occasions.

The same fans would also have savoured the scenes of other topless muscular soldiers on their routine morning jogs.

Song Hye Kyo's Dr Kang is among the admirers of those hunky soldiers in the show.

In real life, the actress says, the cast wanted the scenes to end as soon as possible. She reveals: "It was very cold when the soldiers shot the scene as it was winter. They had a really hard time. We wanted to wrap up the filming quickly for the sake of the boys."

Although pretty boy Song Joong Ki - who was clothed during the jogging scenes - felt sorry for his on-screen subordinates, he too confesses: "On the other hand, looking at those muscular actors made me jealous."

•Descendants Of The Sun is available on viu.com and the Viu app, which can be downloaded from the Apple App Store and Google Play Store. New episodes are available as soon as eight hours after the telecasts in South Korea on Wednesday and Thursday.