World No 1 Tiger Woods said on Monday he is making very slow progress recovering from surgery for a pinched nerve and is unsure when he will make his return to golf.

Woods updated his status on his website, saying he remains sore after the operation and regrets not being able to defend his title at the US PGA Players Championship this week.

"It's tough to miss any tournament, but especially one that's so important to the players and where I'm defending," said Woods, who has been unable to resume his workout regimen because of the March 31 operation, which kept him from playing in last month's Masters.

"My recovery from microdiscectomy surgery for a pinched nerve in my back is coming along, but it's a very slow process," he said. "I'm still sore - not from the procedure itself but from the incision. I just need to get back to my day-to-day activities."

In listing potential timetables for his return, Woods, a 14-time major champion chasing the alltime record of 18 won by Jack Nicklaus, cast doubt upon his ability to play in next month's US Open at Pinehurst, North Carolina,

"As for my return to golf, I really don't know," he said. "I'm doing everything I can and listening to my doctors and working on a strength program and then we just have to see how my back is.

"Some people heal in three months, some people take four months, some people take longer. I just don't know."

The earliest of those spans would bring Woods back in time for the British Open at Hoylake, where he has won before.

The latest of those time frames would have him missing all four of the season's majors, including the PGA Championship at Valhalla, where he has also won a major title.

"I hope to be back sometime this summer, but I just don't know when," Woods said. "There are a lot of big tournaments coming up.

"I haven't used a sand wedge yet. I've just done putting and chip-and-runs using the same length of motion. I haven't really rotated yet.

"As far as taking a full swing, I have conference calls with my doctors every couple of weeks to see how my progress is and just kind of chart it out from there. It's tedious because it's little rehab stuff, but I still have to do it."

The 38-year-old American said keeping fit over his career has helped him build a strong base to recover and having gone through several knee surgeries has given him experience in navigating rehabilitation efficiently.

But even he and swing coach Sean Foley are uncertain what alterations Woods might need to help protect his back and knees when he returns.

"Once I begin swinging a club again, I'm not sure if I will have to make any changes to protect my back," Woods said.

"As far as limitations, it's a building process, just like when I came back from my knee and Achilles injuries. We have to make sure my back heals fine and I have the strength and mobility going forward."

It doesn't hurt to have his girlfriend, US ski star Lindsey Vonn, doing her own rehab regimen alongside, Woods said. Vonn, who missed the Sochi Olympics in February due to a right knee injury, should be back before the end of the year, Woods said.

"It does help to rehab with Lindsey, but her programs are much further along than mine," Woods said. "That does help when you're not the only one suffering. It's a good and bad thing that we're both rehabbing at the same time.

"Her knee is getting stronger and it's good to see. She hopes to be ready to compete again in December."

Woods congratulated Bubba Watson on winning his second crown in three tries at Augusta National last month, saying missing the Masters for the first time was "tough" but "wasn't as hard for me as you might think".