Rookie Joshua Karty hit a 37-yard field goal with 2 seconds to play, and the Los Angeles Rams staged a spectacular late rally for a 27-24 victory over the San Francisco 49ers on Sunday.

Kyren Williams rushed four yards for his third touchdown of the day with just under two minutes left for the Rams, who came back from a 14-point deficit in the second half for their second straight win over San Francisco after losing 10 of the last 12 meetings with their NFC West rivals.

Wide receiver Jauan Jennings set career highs with 11 catches for 175 yards and three touchdowns for the 49ers, who blew a lead of at least 14 points for the first time since 2018.

Los Angeles trailed 24-14 midway through the fourth quarter, but kicker Jake Moody missed a 55-yard field goal attempt for San Francisco with just over two minutes to play.

49ers wide receiver Ronnie Bell then dropped a long pass with one minute to play that would have put San Francisco in field goal range, and Xavier Smith took his first career punt return 38 yards to midfield moments later.

Quarterback Brock Purdy passed for 292 yards for the visitors, but the defending NFC champions and Super Bowl favourites took their second straight road loss after falling at Minnesota last week.

Meanwhile, Kansas City star quarterback Patrick Mahomes threw a pair of touchdown passes, as the Chiefs' defence made a late stand to keep the them unbeaten with a 22-17 victory over the Atlanta Falcons.

The two-time reigning Super Bowl champion Chiefs have been far from perfect, but they won their first three games of the season by a combined 13 points.

Coming off an upset victory at Philadelphia, the Falcons had a couple of shots at the go-ahead touchdown in the closing minutes. (AP)