White circles extraction from a little darker background

I'm trying to extract three white circles on the top left corner from the image below:

I've tried to use:

clone = img.copy()

# Threshold grayscaled image to get binary image
ret,gray_threshed = cv2.threshold(img,90,100,cv2.THRESH_TRUNC)
cv2.imshow('gray', gray_threshed)

# Find edges
edge_detected_image = cv2.Canny(gray_threshed, 350, 100)

# Find contours
contours, _= cv2.findContours(edge_detected_image, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

contour_list = []
for contour in contours:
    contour_list.append(contour)

# Draw contours
cv2.drawContours(clone, contour_list, -1, (255,0,0), 2)

#Resize image
clone = cv2.resize(clone, (800, 800)) 

cv2.imshow('Objects Detected',clone)

The result was:

I've also tried sth like:

img = cv2.blur(img,(5, 5))    

img = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
cimg = cv2.cvtColor(img,cv2.COLOR_GRAY2BGR)

circles = cv2.HoughCircles(img,method=cv2.HOUGH_GRADIENT,dp=1,minDist=2,param1=50,param2=60,minRadius=0,maxRadius=300)

circles = np.uint64(np.around(circles))
for i in circles[0,:]:
    cv2.circle(cimg,(i[0],i[1]),i[2],(0,255,0),2)
    cv2.circle(cimg,(i[0],i[1]),2,(0,0,255),3)

But it showed the error due to the fact that it hadn't found any circles.

Any hints how to solve my kind of problem are welcome. Thanks in advance.