Tension between the United States and Iran is intensifying despite signs, however weak it may be, of negotiations.

On Tuesday, the Iranian government said it had written to the US seeking an apology for alleging that Iran backed a plot to assassinate Saudi Arabia's ambassador to the US on American soil. Teheran's riposte was in response to a letter from Washington offering talks.

Last month, the US said that Manssor Arbabsayara, a 56-year-old US citizen holding both Iranian and US passports, and Gholam Shakuri, a member of Iran's Islamic Revolutionary Guard Corps, were charged with sponsoring and promoting terrorism, including plotting to assassinate the Saudi ambassador. Arbabsayara was arrested in the US, while Shakuri remains presumably in Iran.

The US quickly waged press and diplomacy "wars" against Iran to win the international community's support for its "cause", while Teheran fought back saying Washington's allegation was an "evil plot" aimed at isolating Iran.

We don't know which side is lying. But it is apparent that the US is sensationalizing the allegation with a motive. Washington's political intention behind the move is clear, too, considering the complicated situation in the Middle East.

The US and its Western allies got deeply involved in the political storm that began sweeping across countries in North Africa and the Middle East earlier this year. But Iran, an eyesore for the US, has emerged almost unscathed. Besides, the West thinks Iran used the precious opportunities to "develop its nuclear and missile programs", and make huge profits from rising oil prices during the Libya civil war.

Moreover, the political changes in the Middle East and North Africa, such as the stepping down of pro-US leader Hosni Mubarak in Egypt, will change the majority of Arabian countries' attitudes toward the US and Israel. Israel already seems to be isolated in the Middle East and even faces further confrontation from other countries in the region, which will eventually abate the US' influence in the region. And contrary to US politicians' expectations, Iran became the biggest winner of "Arab Spring".

No wonder, the US chose the "assassination plot" - a most suitable case - to demonize Iran. Given Iranian intelligence agencies' record of having formerly assassinated hostile individuals abroad, and that Adel al Jubeir, the Saudi Arabian ambassador to the US, is antagonistic to Iran, Washington thought it would be easy to gain support of American as well as international public if it portrayed Teheran as an Al-Qaida-like group that plots terrorist attacks on US soil.

Another objective of the US was to set Iran and Saudi Arabia at loggerheads. For long, the two countries have had disputes but no actual conflict. Even after Saudi troops entered Bahrain to help crack down on protesters, Iran maintained restraint. It seems that Iran doesn't want a face-off with Saudi Arabia yet.

The allegations of the terror plot have driven a wedge between the two countries - Saudi Arabia has thanked the US for preventing the attack, while its officials and their US counterparts are considering sanctions and other punitive measures against Iran. In fact, the roots of the intensified conflict between Iran and the US lies in their power struggle in the Middle East.

The West thinks Iran, as a regional power, wants to challenge the US' hegemony in the region and, hence, keeps developing its military capacity by projecting its power to neighboring countries such as Iraq, Afghanistan, Lebanon and Palestine to diminish, rather end, the US' influence in the Persian Gulf and the Middle East.

The US expects to not only prevent Iran from becoming a regional power, but also to overthrow its Islamic regime if possible. One of the US' tactics is to mobilize the international community to pressurize Iran by making use of its nuclear issue.

Since the outbreak of "Arab Spring" earlier this year, the US and Iran both have tried to guide the movement to suit their own interests.

The intention behind what the US has done - abandoning Tunisian leader Zine al-Abidine Ben Ali, Egypt's Hosni Mubarak and Yemeni President Ali Abdullah Saleh, helping hunt down Libyan leader Muammar Gadhafi, supporting Bahrain to stabilize, and pressurizing Syrian President Bashar al-Assad - is actually aimed at strengthening its strategic role in the region and reducing the adverse impact on Israel rather than "promoting democracy in Greater Middle East". One of the US' important goals is to extend the anti-government wave to Iran and Syria.

Iran, on its part, is making efforts to direct the Arab movement to the West by encouraging Arabian countries, under the slogan of "Islamic awakening", to oppose the US and Israel and support Syria.

The US and Iran are actually competing for the dominant role in the Middle East. For the past 30 years, or since the establishment of Islamic regime in Iran in 1979, the cold war between Washington and Teheran has seen no let up. So Washington's allegation against Teheran for plotting the Saudi ambassador's assassination is a prelude to tougher US actions against Iran. And with the situation in Syria intensifying, the US and Iran are likely to prick each other more furiously in the future.

The author is a researcher at the Strategy Research Center of China International Studies Research Fund and China's former ambassador to Iran.