Iran's Supreme Leader Ayatollah Ali Khamenei has praised China for standing by Tehran while it was under international sanctions, saying the Islamic Republic never trusted the West.

Khamenei's comments on Saturday came as the two countries agreed to increase bilateral trade to $600bn over the next decade after a visit to Tehran by Chinese President Xi Jinping.

Iran was keen to expand ties with "more independent countries", Khamenei told Xi, adding the US was "not honest" in the fight against armed groups in the region.

"The Islamic Republic will never forget China's cooperation during the sanctions era," he said.

"Westerners have never obtained the trust of the Iranian nation. The government and nation of Iran have always sought expanding relations with independent and trustful countries like China."

'New chapter'

Iran emerged from years of economic isolation this month when the United Nations' nuclear watchdog ruled it had curbed its nuclear programme, clearing the way for the lifting of UN, US and European Union sanctions.

Xi, whose three-nation regional tour has already taken him to Egypt and Saudi Arabia, Iran's regional rival, became the first Chinese leader to visit the Islamic Republic in 14 years.

"The China-Iran friendship ... has stood the test of the vicissitudes of the international landscape," Xi said, according to the Xinhua news agency.

Following a meeting with Iranian President Hassan Rouhani also on Saturday, Xi said he hoped to open a "new chapter" in relations with Iran in the post-sanctions era.

"In cooperation with the Iranian side and by benefiting from the current favourable conditions, China is ready to upgrade the level of bilateral relations and cooperation so that a new chapter will start in bilateral relations," Xi said according to Iranian state TV.

Cautious approach

China is Iran's biggest trade partner and had continued buying oil from Iran after nuclear-related sanctions were tightened in 2012, despite US pressure.

Trade between the two countries stood at some $52bn in 2014, but that figure dropped last year due to plunging oil prices.

Officials from Iran and China signed 17 documents and letters of intent to broaden bilateral cooperation in energy, industry, transportation, railways, ports, new technology, tourism and the environment.

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Mohsen Milani, executive director of the Center for Strategic and Diplomatic Studies and professor of politics at the University of South Florida, said it was not surprising that Xi visited Tehran after the opening of Iranian markets.

"China has been a good friend of the Islamic Republic during the times that crippling sanctions were imposed on Iran," he told Al Jazeera.

"And because China is a trusted friend of Iran, the Chinese president is trying to take advantage of the opening of the markets and make huge investments in Iran - and Iran is willing to also cooperate with China."

China depends on Iran for 10-15 percent of its energy needs, Milani added. Therefore, it is "extremely important" for Beijing to maintain good ties with Tehran.

"But China is very careful, very cautious not to allow the 'cold war' between Iran and Saudi Arabia to become an obstacle for China to have a good relationship both with the Islamic Republic of Iran and with Saudi Arabia, and I think from a Chinese national interest perspective this is a very sound and logical policy to have."

Regional rivalry

Also on Saturday, Rouhani said the two countries had agreed to cooperate on "terrorism and extremism in Iraq, Syria, Afghanistan and Yemen".

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His comments came after China signalled its support for Yemen's government, which is fighting Houthi rebels said to be backed by Iran, during Xi's visit to Riyadh earlier this week - but Milani said this will not affect its ties with Tehran.

"China has tried to separate its economic policies from political policies, and, therefore, what they are doing in Yemen is not surprising, and I don't think it's going to have any impact whatsoever on its good relationship with Iran and its good relationship with Saudi Arabia," he said.