Prime Minister Narendra Modi will address a high-level summit on sustainable development hosted by the UN on September 25 and will also travel to San Francisco during his US visit, becoming the fourth Indian premier to visit America's West Coast.

The United Nations summit for the adoption of the post-2015 development agenda will be held from September 25 to 27 here and will be convened as a high-level plenary meeting of the General Assembly.

Slated to be one of the largest ever gatherings at the UN, more than 150 world leaders are expected to attend the Sustainable Development Summit to formally adopt the outcome document of the new agenda.

Modi is expected to address the gathering on September 25 after which he will head to the West Coast to visit San Francisco, sources told PTI.

The 70th session of the General Assembly will open on September 15.

The annual high-level General Debate will begin on September 28 and will run through October 6.

Since Modi is likely to return to India around September 28, External Affairs Minister Sushma Swaraj is expected to represent the country at the General Debate.

Modi is expected to travel to San Francisco after his address to the Sustainable Development Summit on September 25, becoming the fourth Indian Premier to visit US' West Coast.

Modi's proposed visit to San Francisco would also revive -- after a gap of four decades with the exception of the former prime minister P V Narasimha Rao in 1994 -- the post-independence tradition of Indian prime ministers visiting the US cities other than New York or Washington DC.

The thriving Indian Diaspora in the US, in particular those on the West Coast and the Silicon Valley has welcomed Modi's decision to visit San Francisco and preparations are in full swing to accord a grand welcome to him at the SAP Center in San Jose.

Modi had given his maiden address to the UN General Assembly last year and had then travelled to Washington to meed US President Barack Obama.