US Secretary of State Hillary Clinton favoured arming Syria's rebels early in that country's civil war but was overruled by President Barack Obama, the former secretary of state said in her new memoir obtained by CBS News.

Clinton's long-awaited book Hard Choices - seen as an unofficial start to her expected presidential run - is set for a June 10 release, but CBS News said Thursday it purchased a copy at a bookstore.

"Wicked problems rarely have a right answer; in fact, part of what makes them wicked is that every option appears worse than the next. Increasingly that's how Syria appeared," she wrote.

Clinton said that she returned to Washington from an overseas trip convinced that arming and training moderate Syrian rebels was the best move to turn the tide against President Bashar al-Assad.

"The risks of both action and inaction were high, (but) the president's inclination was to stay the present course and not take the significant further step of arming rebels," she added.

"No one likes to lose a debate, including me. But this was the president's call and I respected his deliberations and decision," she wrote, according to CBS News.

The book touches on scores of key policy issues including the Iraq war, Benghazi, as well as fallout from the Arab Spring.

But the Syria episode illustrates a key break between Clinton, who is mulling new presidential bid after losing the Democratic nomination to Obama in 2008, and a president whose popularity has fallen.

"From the beginning of our partnership, he had promised me that I would always get a fair hearing. And I always did. In this case, my position didn't prevail," she wrote.

She also wrote about how she eventually realized that her vote supporting the Iraq war resolution in 2002 when she was a US senator was a grave mistake.

"I thought I had acted in good faith and made the best decision I could with the information I had. And I wasn't alone in getting it wrong," she wrote. "But I still got it wrong. Plain and simple."

The civil in Syria is seen to be further fuelled by the involvement of many external sides to the conflict, including Russia, the United States, Iran and Saudi Arabia, amongst others.

Turkey to flag Iran's support for Syria during Rouhani visit

Meanwhile, Turkish officials intend to criticise Iran's support for Syria when Iranian president Hasan Rouhani visits Turkey next week, a ruling party official said Thursday.

Rouhani is scheduled to visit Turkey on Monday, his first visit to the country since he was elected last year.

"One topic we will bring up to Rouhani will be Iran's position regarding the Syrian civil war," said Yasin Aktay, deputy chairman of the ruling Justice and Development (AK) Party.

Iran, along with Russia and China, is one of the strongest backers of Syrian dictator Bashar Assad.

"We will remind him that the policy they have followed over Syria is not going to benefit Iran - something that we will tell him directly," Aktay said.

Aktay answered questions from foreign correspondents during breakfast at AK Party headquarters in Ankara, Turkey’s capital.

"During the Arab Spring, Iran was popular in the region, and Iran would have benefited from this period," Aktay said. "Yet their popularity among the Muslim countries has decreased due to their policies regarding Syria."

Turkey, which has been supportive of the Syrian uprising and is a host to almost one million Syrian refugees, is itself worried about the danger posed by certain extremist elements in Syria.

The Turkish Council of Ministers on Tuesday designated Al Nusra Front - a branch of Al-Qaeda operating in Syria - as a terrorist organization.

The decision comes amid mounting Western concerns over the possible threat to Europe and the United States posed by some foreign fighters returning from Syria, and amid American pressure for tighter Turkish control of its borders with Syria.