U.S. Secretary of State John Kerry on Sunday called for a rapid expansion of U.S.-India trade and commercial ties as he attended an international investment conference ahead of visit by President Barack Obama later this month.

Kerry led the U.S. delegation to the investment summit in Ahmedabad, the main financial city in Indian Prime Minister Narendra Modi's home state of Gujarat. Kerry said it was imperative that economic ties between the two countries grow for the sake of Indian development, reducing poverty and fighting climate change.

"We can do more together, and we must do more together, and we have to do it faster," Kerry told the conference, which Modi opened.

Commerce and investment between the United States and India have jumped in recent years, growing nearly five-fold since 2000 with bilateral direct investment at almost $30 billion. But it still faces obstacles dues to restrictive Indian laws and regulations.

U.S. officials said they were working on agreements that could include a solar energy deal, a plan to improve rural electrification and potentially a carbon reduction pact that they hoped could be signed when Obama visits India to participate in India's annual Republic Day ceremonies on Jan. 26.

Modi has invited Obama to be the "chief guest" at the celebration, the first sitting American president to be so honored.

In Ahmedabad, Kerry also planned talks with the prime minister of the tiny Himalayan kingdom of Bhutan, marking a rare cabinet-level meeting between the two nations.

Kerry stopped briefly in Germany on Saturday to meet with the ailing ruler of Oman, the Mideast country that's served an important intermediary role in negotiations over Iran's nuclear program.

Kerry spent about 90 minutes with Sultan Qaboos bin Said, 74, who has been receiving medical treatment in Germany since November. Kerry later tweeted that he was "grateful for strong friendship" with the sultan.

Among the topics they covered were the Iran talks, which are set to resume this coming week in Switzerland; the situation in Yemen; the Syrian civil war; the terrorist attacks in Paris; and tensions between Israelis and Palestinians.

Oman hosted several secret meetings between U.S. and Iranian officials in the months leading up to an interim nuclear deal in November 2013.