Ahead of the formal easing of international sanctions on Tehran set for the beginning of 2016, tensions have mounted.

On Thursday, Iranian President Hassan Rouhani ordered his defense minister to expand Iran's missile program, in

response to a US threat to impose new sanctions over a ballistic-missile test Iran carried out in October.

Iranian President Hassan Rouhani talks to journalists after he registered for February's election of the Assembly of Experts, the clerical body that chooses the supreme leader, at Interior Ministry in Tehran December 21, 2015. Reuters The White House has delayed imposing new financial sanctions on Iran, Reuters reports.

"As the US government is clearly still pursuing its hostile policies and illegal meddling ... the armed forces need to quickly and significantly increase their missile capability," Rouhani wrote in a letter to Defense Minister Hossein Dehghan, published on the state news agency IRNA.

Iran condemned the new sanctions on international companies and individuals over Tehran's ballistic-missile program.

"As we have declared to the American government ... Iran's missile program has no connection to the (nuclear) agreement," state television quoted Foreign Ministry spokesman Hossein Jaber Ansari as saying.

The US began preparing the sanctions, which target nearly a dozen companies and individuals in Iran, Hong Kong, and the United Arab Emirates, for their suspected role in developing Iran's missile program and for supporting human-rights abuses and international terrorism, The Wall Street Journal reported, citing US officials.

According to The Journal, the White House was recently warned by Iranian officials that new sanctions would be considered by Supreme Leader Ayatollah Ali Khamenei as a violation of the brokered nuclear agreement.

The US, however, maintains that the US Treasury Department retains the right to blacklist Iranian entities despite the July agreement, arguing that areas of missile development, terrorism, and human rights are separate from the nuclear deal.

"Iran will resolutely respond to any interfering action by America against its defensive programs," Jaber Ansari said, rejecting the new sanctions as "arbitrary and illegal."

This dispute comes after Iran and six world powers, including the US, reached a historic nuclear deal in July that will remove certain US, EU, and UN sanctions on Tehran in exchange for Iran's accepting curbs on its nuclear program.

Iran's controversial ballistic-missile test

An Iranian Emad rocket is launched as part of a test at an undisclosed location. Thomson Reuters

According to a confidential report seen by Reuters on December 15, Iran fired a medium-range ballistic Emad rocket on October 10 that was capable of delivering a nuclear warhead.

This month, a UN Security Council panel ruled that Iran violated paragraph nine of Security Council resolution 1929 by test-firing that rocket.

Iran, however, says the resolution bans only missiles "designed" to carry a nuclear warhead, not ones "capable of" doing so.

Iran has called the Emad missile a "conventional missile."

Since the October 10 incident, Iran was criticized for conducting a live-fire training exercise near a US aircraft carrier in the Gulf.

Live-firing exercise near an American aircraft carrier

The aircraft carrier USS Harry S. Truman in the Straight of Hormuz. Flickr/US Navy Photo Iran denied on Thursday that its Revolutionary Guards launched rockets near the US aircraft carrier Harry S. Truman in the Gulf on Saturday and condemned US plans for new sanctions over its ballistic-missile program.