TATSUNO, Japan (REUTERS) - Japanese Prime Minister Shinzo Abe could soon hold a summit meeting with China's president, Xi Jingping, an adviser to Abe said on Sunday, adding that he had met senior officials close to the Chinese leader on a secret visit to Beijing this month.

Abe, whose ruling bloc cemented its grip on power with a win in upper house of parliament elections a week ago, called on Friday for an unconditional meeting between his country and China as soon as possible.

Sino-Japanese ties, often fragile, have been seriously strained since September when a territorial row over tiny islands in the East China Sea flared.

Concern that the conservative Japanese leader wants to recast Japan's wartime history with a less apologetic tone has added to the tension.

While Abe's call last week drew a cool reaction from Beijing, his adviser, Isao Iijima, said Chinese leaders were considering the call and he believed they would respond positively.

"I feel they are troubled by it, they are deeply thinking about it," Iijima told reporters when asked about the summit call.

"I don't think it will take that long (before they meet)," he said on the sidelines of a speech in his hometown in central Japan.

Iijima, who made a surprise visit to North Korea in May, declined to identity of the people he met in China or give details of the discussions he said were held over two days in mid-July.

"I went there to ask them what they really think," Iijima said, stressing that the visit was private.

China, in responding to Abe's call, said its door was always open for talks but Japan should "stop using empty slogans about so-called dialogue to gloss over disagreements".

Iijima said his hope for a summit stemmed from the "atmosphere" of his meetings.

He said he did not discuss matters related to North Korea.

On Friday, Japan's Defence Ministry issued a policy report repeating concerns about China's military build-up and its activities near the disputed islands.

Iijima also reiterated his belief that leaders of Japan and North Korea should hold a summit, and that pressure on North Korea without diplomacy would not help Japan solve the issue of citizens kidnapped by the secretive regime decades ago.

"The door has been closed for the last ten years ... we can't just apply pressure, we also should talk," Iijima said, referring to North Korea.

"At least the leaders of the two countries should meet." North Korean agents kidnapped Japanese citizens decades ago to help train spies.

Abe owed much of his popularity when he first took office in 2006, at the beginning of a first term in power, to his tough stance towards North Korea over the abductees.

Five of the abductees were repatriated and North Korea has said another eight died. Japan has been pressing for more information about the eight and others it says were also kidnapped. It wants any survivors sent home.

Japan started looking into resuming inter-governmental talks with North Korea after Iijima's surprise visit in May, Japanese media said following the visit.

Talks were last held in November 2012 but have been halted because of North's missile launch in December and nuclear test in February.