Sanders has secured 29.7 percent of the vote in Iowa, according to his campaign's own results from almost 40 percent of the precincts in the state.
Earlier in the day, another Democratic candidate Pete Buttigieg claimed the victory.
"Iowa, you have shocked the nation. By all indications, we are going on to New Hampshire victorious. #IowaCaucuses", Buttigieg wrote on Twitter.
Candidates at Democratic primaries are allocated a certain number of delegates after each electoral event. After all the elections are finished, the candidate with the most delegates becomes the Democratic party's presidential candidate.
The outcome of the Iowa caucuses is historically more predictive for Democrats. In case of contested races they have awarded the nomination to seven out of 10 state victors since 1972, but only two of them - Jimmy Carter and Barack Obama - made it to the White House.
Among Republicans, three of eight winners have received the national nomination with just one, George W. Bush, eventually becoming US President.