U.S. President Donald Trump speaks to reporters at the White House in Washington D.C., the United States, June 22, 2019. He said the United States would impose major additional sanctions against Iran. (Xinhua/Ting Shen)

WASHINGTON, July 10 (Xinhua) -- U.S. President Donald Trump on Wednesday made new threats against Iran by saying that sanctions will be added "substantially" to the Middle East country as bilateral tensions remain high.

"Iran has long been secretly 'enriching,' in total violation of the terrible 150 Billion Dollar deal made by John Kerry and the Obama Administration. Remember, that deal was to expire in a short number of years. Sanctions will soon be increased, substantially!" Trump wrote in a morning tweet.

Trump's words came days after Iran announced that it had raised the concentration of its enriched uranium to 4.5 percent from 3.67 purity, a limit set by the 2015 Iran nuclear deal for Iran's enrichment concentration.

Following Tehran's announcement, U.S. Vice President Mike Pence, along with other senior Trump administration officials, vowed on Monday to continue to pile up economic pressure on Iran.

Ties between the United States and Iran have deteriorated significantly after Washington walked away unilaterally from the landmark nuclear pact in May 2018 and reimposed energy and financial sanctions, which had been removed under the deal, on Iran.

The International Atomic Energy Agency (IAEA) chief announced in March that Iran was complying with the nuclear deal made with major countries in 2015 aimed at preventing Tehran from building nuclear weapons.

Recently, Iran started to suspend parts of its commitments subject to the nuclear deal and threatened to go further. Iran's Foreign Ministry spokesman Abbas Mousavi said on Wednesday that the country keeps its door open to diplomacy and talks over its nuclear issue, Press TV reported.